HBW 10 - Family text: Troglodytidae (Wrens)
Family TROGLODYTIDAE (WRENS)
- Very small to medium-sized, mostly insectivorous birds with finely pointed and often decurved bill, tail variously long and graduated to extremely short, one species with sturdy legs and bill; plumage colours mostly grey and brown, often with rufous tones.
- 9-22 cm.
- New World, one species also in Eurasia and north Africa.
- Forest, forest edge, scrubland, suburban gardens, marshes, desert brushland, rock faces.
- 17 genera, 85 species, 401 taxa.
- 6 species threatened; none extinct since 1600.
Troglodytidae is essentially a New World family, most diverse in Central America and north-western South America, with only one species, the Northern Wren (Troglodytes troglodytes), having escaped to the Old World. Everywhere, the members of this family are small, highly active, and highly vocal insectivores, perhaps seldom seen but always, it seems, dominating the soundscape.
The relationships of wrens to other avian groups have been debated over the years. In 1881, R. B. Sharpe, in the Catalogue of the Birds in the British Museum, classified the wrens as a subfamily within the Timaliidae, which also included the dippers (Cinclidae). A few years later, the American Ornithologists' Union, in its 1889 Check-list of North American Birds, included the wrens as one subfamily and the mockingbirds and thrashers (Mimidae) as another within the family Troglodytidae, which it placed just after the wagtails and pipits (Motacillidae) and the dippers, and before the treecreepers (Certhiidae) and the tits and chickadees (Paridae). In 1904, R. Ridgway discussed the long-suspected probability that other songbirds most closely related to the wrens were the treecreepers and the mimids, "not far removed" from the dippers. Ridgway used the relatively high degree of cohesion between the anterior toes to show that the wrens were more like the treecreepers and parids than like the mimids, the latter, which are often referred to as "mimic-thrushes", being in turn more like the true thrushes (Turdidae). E. Mayr and D. Amadon, in 1951, placed the wrens in the family Muscicapidae, which they subdivided into the seven subfamilies Muscicapinae, Timaliinae, Sylviinae, Malurinae, Turdinae, Miminae, and Troglodytinae. They thereby suggested a relationship with, among others, the Old World flycatchers (Muscicapidae), the Old World warblers (Sylviidae) and the Australasian fairywrens (Maluridae). Another group that has often been associated with the wrens is the nuthatches (Sittidae). In a typical linear classification, Troglodytidae has been placed between the dippers and the mimic-thrushes, as by Mayr and Greenway in 1960, or between the treecreepers and the dippers, as by Ridgway in 1904.
In more recent decades, the data obtained by C. G. Sibley and J. E. Ahlquist from studies involving DNA-DNA hybridization suggested a novel hypothesis for the relationships of wrens. Their results indicated that the wrens, including the highly unusual Donacobius (Donacobius atricapillus), were the sister-group to the gnatcatchers (Polioptila), the gnatwrens (Microbates) and the Verdin (Auriparus flaviceps). The first two of these groups had traditionally been located within the subfamily Polioptilini of the Muscicapidae, while the Verdin had been included with the penduline tits in the family Remizidae. Furthermore, Sibley and Ahlquist suggested that the wrens, the gnatcatchers and the Verdin together formed a sister-group to the treecreepers of the genera Certhia and Salpornis, and proposed that they all be combined into a single family, the Certhiidae. This family, thus expanded, was a sister-group to Sittidae, which comprises the nuthatches (Sitta) and the Wallcreeper (Tichodroma muraria). In addition, these genetic data emphasized the groups to which the wrens appeared not to be closely related: the babblers, which were moved to the family Sylviidae, the mimic-thrushes, which were found to be a sister-group to the traditional starling family Sturnidae, placed in another superfamily, and the dippers, forming yet another superfamily.
These recent ideas concerning the relationships of wrens received considerable support from a subsequent genetic analysis undertaken by F. H. Sheldon and F. B. Gill, whose results were published in 1996. It was agreed that the wrens' closest relatives are the gnatcatchers and gnatwrens, the treecreepers and the nuthatches. These authors' data also supported Sibley and Ahlquist's hypothesis that the mimic-thrushes and Sturnidae, the starlings, are sister-groups, well separated from the wrens. The Verdin, however, remained with the penduline tits.
Many of these proposed relationships were, in turn, corroborated in 2002 by F. K. Barker and colleagues, and they seem now to be generally accepted. Thus, the sister-group to the wrens is Polioptila, and the two together are sister to the treecreepers. Somewhat less certain is that all of these birds are the sister-group to the nuthatches. Further, all of these groups appear together to represent a sister-group to a clade that includes the dippers, the starlings, the mimic-thrushes, the thrushes of the genus Turdus, and the Old World Muscicapa flycatchers.
Over the last century or so, the family itself has largely been accepted as comprising a cohesive, monophyletic group of species, once several members of the Timaliidae and the Sylviidae were removed. The one remaining problem, however, concerns the Donacobius, a puzzling marsh-nesting bird found throughout most of lowland tropical South America. Details of its anatomy, social structure and behaviour (see General Habits) have suggested that it is either a wren or a mimic-thrush, and classifications over the years have alternately placed this species in either the Mimidae or the Troglodytidae. One of the last public statements, in 1982, declared that the Donacobius is a wren, and not a mimid, a view based on a still unpublished study of pterylosis by M. H. Clench and colleagues. That declaration was accepted by the American Ornithologists' Union in 1983. A year later, the genetic data of Sibley and Ahlquist indicated that the Donacobius was, in fact, as genetically distant from the wrens as it was from the mimic-thrushes, although, somewhat surprisingly, Sibley and Ahlquist used this equivocal information to place the Donacobius among the wrens. More recent genetic analyses by Barker, however, suggest that this species is neither a wren nor a mimid but, rather, a unique "sylvioid", a group that includes the babblers and the Old World warblers, as it clusters close to Prinia and Zosterops.
For the time being, it is deemed best to retain the Donacobius among the Troglodytidae, pending its "official" transfer to a more appropriate taxonomic position.
Less effort has been expended in trying to determine the relationships among the genera within the wren family. Some debate has centred on the genera Hylorchilus, Catherpes and Salpinctes. In 1904, for example, Ridgway accepted all three genera, but in 1960 Mayr and J. C. Greenway placed the Canyon Wren (Catherpes mexicanus) with the American Rock Wren (Salpinctes obsoletus) in the genus Salpinctes. Sumichrast's Wren (Hylorchilus sumichrasti) was originally described in the genus Catherpes back in 1871, and then moved to Hylorchilus in 1897, although, 90 years thereafter, J. W. Hardy and D. J. Delany argued that, on the basis on vocal similarities, it should be returned to Catherpes. A vocally distinct former subspecies of Sumichrast's Wren was elevated to full species status in 1993, becoming Nava's Wren (Hylorchilus navai), and thereby providing a small point in favour of keeping these two closely related wrens in their own separate genus, Hylorchilus.
A recent survey of the wrens in the genus Troglodytes, carried out by N. H. Rice and colleagues, is consistent with some suspected relationships, but it also offers some surprises. Most of all, it encourages further study not only of this genus but also of others within the family, especially the speciose Thryothorus. The genetic data obtained by Rice and co-workers indicate, not surprisingly, that the Northern House Wren (Troglodytes aedon), the Southern House Wren (Troglodytes musculus) and the Brown-throated Wren (Troglodytes brunneicollis) represent one major clade and, perhaps less expectedly, that the Rufous-browed Wren (Troglodytes rufociliatus) is a sister to this clade; in their analysis, the two house wrens are sister-taxa. An association of the same group of three species had been found in the analysis made just a few years earlier by R. T. Brumfield and A. P. Caparella, but these authors concluded that the Northern House Wren and the Brown-throated Wren were sister-taxa, a disagreement that will have to be settled through further analyses. Another major clade identified by Rice and colleagues included the Mountain (Troglodytes solstitialis), Tepui (Troglodytes rufulus) and Ochraceous Wrens (Troglodytes ochraceus). Surprisingly, the sister-group to these two clades was the Timberline Wren (Thryorchilus browni), and the Northern Wren fell well outside this grouping and was considered sufficiently different to be placed alone in a monotypic genus, Nannus. These various hypotheses of relationships are intriguing, but they are based on relatively limited genetic sampling, and the true relationships of these species to one another must remain a mattter of debate until further study is undertaken.
The only other proposed relationships among wren genera are those implied in various linear classifications, such as the sequence that is used in the present treatment. In such linear sequences, the genera Troglodytes and Thryorchilus, for example, are placed together, implying that they are closely related, but few data exist to support one linear classification over another.
Further genetic analyses, such as those by Barker, are likely to clarify relationships among the genera. In his thorough study, Barker drew genetic samples from 20 species among the traditional Troglodytidae, and from all genera except the monotypic Ferminia, Thryorchilus and Uropsila, although, as implied in the preceding paragraphs, Thryorchilus is assumed to be closely allied to Troglodytes. This study also incorporated close relatives of the wrens, such as treecreepers, nuthatches, mimids and a gnatcatcher, as well as several more distantly related species.
Barker's results conflict with the traditional linear series of genera, which had hitherto been the "best guess" of a sequence listed from "primitive" genera to "derived" ones. For example, although traditional linear classifications place Campylorhynchus first, and even within its own subfamily, these wrens are clearly not the sister-group to all other wrens, and phylogenetic trees show them nested well within, rather than basal to, the other troglodytids. In addition, such linear series place at the end those genera that tend to be ground-oriented, including Thryorchilus, Henicorhina, Microcerculus and Cyphorhinus, but this group is polyphyletic. Thryorchilus belongs to a clade that also includes Cistothorus and Troglodytes, while Microcerculus may be part of a basal radiation within the family, and Henicorhina and Cyphorhinus are sister-taxa, which in turn are probably sister to most Thryothorus.
Barker also concluded that Hylorchilus and Catherpes are sister-taxa, a result consistent with arguments based on similarity of song between the Canyon Wren and Sumichrast's Wren, with the distinct songs of Nava's Wren secondarily derived. It is possible that the sister-group of these wrens is the similarly petrophilous (rock-dwelling) American Rock Wren, which would permit all four of these species to be combined in the same genus, but other data suggest that Salpinctes, the rock wren, may be basal to all other wrens.
One of the more controversial of Barker's hypotheses is that the Carolina Wren (Thryothorus ludovicianus) is a sister-taxon to Bewick's Wren (Thryomanes bewickii), the two being sister-taxa to the Campylorhynchus wrens. A discovery that Thryothorus is polyphyletic, with, for example, Thryomanes nested within it, would not be so surprising in view of the large number of species currently contained in this little-studied genus, but the removal of the Carolina Wren to so distant a location is a rather novel hypothesis.
Clearly, these genetic data provide hypotheses for relationships that are worthy of further testing. Moreover, and above all, it is important to realize that the above-mentioned ideas are largely hypothetical, representing what are currently the best guesses based on limited data. It may be hoped that further research work will significantly clarify these relationships.
Research on vocalizations (see Voice) has suggested that recognition of additional species within the Troglodytidae may be warranted. The Marsh Wren (Cistothorus palustris), for example, consists of highly differentiated eastern and western populations in North America, as also does the Northern Wren, the Old World population of which may represent a further one or more species. As revealed by D. E. Kroodsma and co-workers, the Sedge Wren (Cistothorus platensis), too, exhibits substantial variation in plumage, vocal behaviour and style of song development, and habitat selection (see Habitat) within its large range, which extends from North America down through South America, and it is almost certain that more than one species is involved.
So far as the Marsh Wren is concerned, not only do the eastern and western populations differ in many perceivable ways, many of these differences being based on genetic differences, but they also occur together in the same marshes in central Saskatchewan, where interbreeding between them is limited. Eastern and western males defend territories against one another, and analysis of mitochondrial DNA in the population largely reveals two types of wren, one eastern and one western, with little interbreeding. If they interbred freely, then one would expect a genetically homogeneous population of Marsh Wrens in this transition zone, but the two types of wren signal that they themselves distinguish each other as two species.
Another currently recognized species in which songs provide clues to past evolutionary history is the Northern Wren. In North America alone there are two strikingly different singers, again separated along east-west lines. Eastern and boreal males occur from Nova Scotia south to North Carolina, and west at least to Michigan and Minnesota, while western males have been documented from British Columbia south to California; it is not known where these two types of singer meet, if they do at all. In addition, the songs of Northern Wrens in the Old World are different from those in North America.
On the basis of singing behaviour alone, the emerging hypothesis is that the western Nearctic and eastern Nearctic Northern Wrens diverged first, followed by a divergence of eastern Nearctic wrens and Palearctic wrens. This hypothesis is now supported and enhanced by the work of Drovetski and co-workers, who analyzed the genomic DNA from 97 wrens from throughout North America and Eurasia. These authors estimate that the western Nearctic wren diverged from the Holarctic ancestor roughly 1·6 million years ago, and that the Holarctic ancestor diverged into eastern Nearctic wrens and Palearctic wrens about one million years ago. Within Eurasia, four more groups are now identified, having originated as follows: eastern and western Palearctic wrens diverged about 0·83 million years ago, and then each respective population diverged one more time, between 0·54 million and 0·67 million years ago. The proliferation of these groups thus occurred during the early and middle Pleistocene glaciations. How many species occur in this Holarctic distribution depends on one's definition of a species, but the best guess is that at least three should be recognized, two in North America and one in Eurasia, and that perhaps each of the four clades in Eurasia could also be recognized as a full species.
Very recently, differences in songs among supposed Grey-breasted Wood-wrens (Henicorhina leucophrys) were used in helping to identify a new species in the western Andes of Colombia. This new species, the Munchique Wood-wren (Henicorhina negreti), is found only at high elevations on the Pacific slope, in the extremely wet, stunted cloudforest that is almost continuously shrouded in fog. In the taller forest at lower elevations on the west slope, it is replaced abruptly by the Grey-breasted Wood-wren of the subspecies brunneiceps, and on the drier east slope by the nominate race of the latter. Compared with the adjacent populations of Grey-breasted Wood-wrens, the Munchique Wood-wren has a more distinctly barred abdomen, a darker juvenile plumage, relatively longer tarsi and a proportionately shorter tail.
It is likely that, were it not for the fact that these wrens learn their songs and therefore have geographically differentiated song dialects (see Voice), far more species of wren would be recognized. New species are rather routinely described for various suboscines, such as the tyrant-flycatchers (Tyrannidae), the songs of which are not learnt and therefore represent a vocal signature for the genotype of the bird; in the case of Thamnophilus punctatus, another suboscine that presumably does not learn its songs, M. L. Isler and colleagues have suggested that what was previously considered a single species, the Slaty Antshrike, in fact consists of six or more species. Most species of songbird, however, offer no such obvious vocal clue as to how their populations are differentiated, and the conservative approach is therefore to keep the highly differentiated Northern Wrens in one species. The same reservations apply to the Marsh Wrens and the Sedge Wrens and, no doubt, to other troglodytids, too. Nevertheless, future research will surely lead to the elevation to species rank of a good number of these behaviourally and vocally distinctive taxa. Indeed, very recently published work suggests that the globally threatened Apolinar's Wren (Cistothorus apolinari) of Colombia may, in fact, consist of two species, differentiated not only morphologically and by habitat preference but also by song. One form, currently described as the subspecies hernandezi, not only has a different song from the nominate race, but additionally, and in contrast to the latter, indulges in communal group song, both antiphonal and in duet.
Wrens are generally small birds. In fact, in the New World, only some of the hummingbirds (Trochilidae), kinglets (Regulus), gnatcatchers and parulid warblers are lighter in weight than the smallest wrens. The largest member of the family, the Giant Wren (Campylorhynchus chiapensis) of southern Mexico, is a giant only in comparison with its relatives. With a length of 22 cm and weighing up to 57 g, it is about the size of a smallish thrush. At the opposite extreme, the Sedge Wren is about 9 cm long, and the Northern Wren weighs as little as 6 g. Birds' weights do, of course, vary greatly according to the condition of the individual.
None of the troglodytids is brightly coloured in plumage, and reds, bright yellows, greens and blues are entirely absent in the family. Instead, all wrens are various shades of brown or rufous, often with areas of black and white. Despite this modesty of coloration, some species, particularly in the genus Thryothorus, have very attractive patterns of plumage, especially around the head and chest. One feature characteristic of virtually all wrens, and largely absent in most other passerines, is the predominance of barring on both the remiges and the rectrices. This usually takes the form of narrow alternating sections of blackish or dark brown and lighter brown on the outer webs of the primaries and secondaries; the tertials and the rectrices are typically barred on both webs. In a few species, especially the ground-dwelling wrens inhabiting thick, wet, dark tropical forest, the barring is very muted or almost absent, but, on close examination, traces are still discernible, suggesting that the ancestral wrens were barred. The plumage of wrens tends to be soft and rather fluffy.
Most members of the family have twelve rectrices, although the Timberline Wren, the two members of the genus Hylorchilus and the enigmatic Zapata Wren (Ferminia cerverai) have only ten. The rectrices tend to be rounded, with the outer ones often significantly shorter than the central ones. The tail length differs widely among genera. In the genus Odontorchilus, containing two South American species that appear to be converging evolutionarily with the unrelated Polioptila gnatcatchers, birds of similar habits and lifestyle, the tail accounts for almost half of the bird's total length. In the cactus wren group, the genus Campylorhynchus, it is almost as long in relative terms. More usually, however, the tail is shorter, and in some genera much shorter. The Northern Wren, for example, has a tail that makes up only 25-28% of the total body length. The most extreme instance, however, is that of the ground-dwelling Microcerculus wrens, which appear almost tailless in the field, the rectrices being typically no more than one-fifth of the total length of the bird. This is a further example of convergent evolution, in this case with the small Grallaricula antpittas in the antbird family (Formicariidae), the habitat and habits of which they share.
All wrens have ten primaries. Although the tenth primary, the outermost, may be much reduced, it is always present. They have relatively short and rounded wings, with usually the middle primaries, P5 to P7, the longest. The primaries of some species, when spread out, seem almost to describe an arc of a circle. The wing shape has clearly evolved for a lifestyle centred around short flights in dense vegetation, rather than for aerodynamic efficiency. Notwithstanding this, some species, notably the Northern Wren and the Northern House Wren, undertake long migratory journeys (see Movements).
There is no sexual dimorphism in wren plumages. Males and females are essentially identical in virtually all cases. In addition, there is frequently little distinction between adults and young. In some members of the genus Campylorhynchus and a few of the genus Thryothorus, juveniles do differ markedly in plumage from the adults of the species, but in most genera the distinctions between adult and young plumages tend to be minor and subtle.
Although the troglodytid bill tends to be long and often substantially decurved, considerable variation exists within the family, the bill morphology being dependent on lifestyle and food preferences. Some genera, such as Salpinctes and Troglodytes, have a very fine bill. Members of Catherpes and Hylorchilus, two genera specializing in foraging on rock faces, have a bill that is especially fine and decurved; they also exhibit several other specific modifications, including a flattened skull, for obtaining prey from crevices in rocks. The two Odontorchilus wrens have a very fine bill with a peculiar little notch, of unknown function, two-thirds of the way down the cutting edge of the upper mandible. In the genus Campylorhynchus, several species of which include major quantities of hard vegetable matter as well as large, well-armoured arthropods in the diet, the bill may be quite heavy and powerful. Some members of this genus, such as the Band-backed Wren (Campylorhynchus zonatus), appear to specialize in probing for prey in epiphytes and, consequently, have a longer, finer bill.
The most peculiar bills in the family are found in the ground-dwelling genus Cyphorhinus. The bill is quite stout and powerful, with a curious upper mandible compressed into a sharp, wedge-shaped ridge along the upper surface, giving an odd "Roman-nose" appearance. The precise function of this character is not known. Another unique feature of this genus, shared with many other, totally unrelated South American suboscine passerines, is a patch of bare, coloured skin around the eye.
Rictal bristles are present in some troglodytid genera, such as Campylorhynchus, Henicorhina and Uropsila. In some cases, as in the three Cyphorhinus species and some Thryothorus wrens, they may be very short or unobvious, but in many genera, including Troglodytes, Cistothorus, Salpinctes, Catherpes, Hylorchilus and Microcerculus, rictal bristles are obsolete or absent.
As may be expected for a group of birds that spends most of its time in foraging near the ground, the legs and feet of wrens tend to be substantial in proportion to the small size of the birds, and in some cases they are rather heavy and powerful, with large claws. In contrast, the legs and feet of the canopy-dwelling Odontorchilus wrens are quite fine and dainty.
Finally, one species currently included in Troglodytidae, but almost certainly belonging elsewhere (see Systematics), differs in several respects from the "true" wrens. In general external appearance, the Donacobius resembles the mimids more than it does any wren. The wings are powerful and rather rounded, with very short primaries, and the tail is long and very graduated, almost fan-shaped in fact, with the outer rectrices much shorter than the central ones. The legs are long and very robust, and the bill is long and sturdy, with rictal bristles present but not very obvious. A unique feature of the Donacobius is the presence of a patch of bare yellow to orange skin on each side of the neck, not usually visible in the field, but distended during display.
Information on moult sequences is generally lacking for the great majority of wren species, namely those occurring in Central and South America; this would be a very fertile and productive area of study. More data are available for those species found in North America and Europe. Typically, the first moult that a bird undergoes, that in which it renews the original feathers that it grew as a nestling, occurs within a few months of fledging and is incomplete. The body feathers are replaced, but usually only some flight-feathers are renewed. The extent of this moult is variable, not only among species but also among different individuals of a single species. In Bewick's Wren, for example, some individuals replace no tail feathers at all in the first moult and some replace all of them, but about two-thirds of individuals replace only some of the rectrices. The more southerly races of this species tend to renew more feathers than do the northern ones. Among wrens, the replacement of the remiges in this first moult is frequently incomplete and often eccentric; rather than starting at the outermost or innermost primary or secondary and proceeding inwards or outwards in sequence, it begins several feathers in, thereby producing a mixture of old and new feathers in patches. In subsequent years, the prebasic moult of all North American wrens is complete.
Most wrens do not undergo a pre-breeding moult, a spring moult that in some families, such as the parulid warblers, results in a characteristic bright breeding plumage. The exceptions to this are the Cistothorus wrens, which have a partial moult before breeding. In the case of the Sedge Wren, this results in the replacement of a variable number of greater wing-coverts, tertials and rectrices, but not, apparently, of the primaries or secondaries. A similar pattern is found in the Marsh Wren.
As a family, the wrens have been highly successful in adapting to a very wide range of habitats, occupying a variety of ecological niches which, in continents other than the Americas, are taken up by several different avian families.
The largest wren genus is Thryothorus, which has the greatest density of species occurring in southern Central America and north-western South America. Within this one genus there is a considerable diversity of habitat requirements, as may be expected since its members are found all the way from southern Canada south to Bolivia, but, in broad terms, all require areas of trees with substantial undergrowth. Some species, such as the Banded Wren (Thryothorus pleurostictus) and the Happy Wren (Thryothorus felix), are found in dry forest and bushland. The majority, however, favour more humid habitat, while some, such as the Bay (Thryothorus nigricapillus), Riverside (Thryothorus semibadius) and Coraya Wrens (Thryothorus coraya), show a preference for the vicinities of actual watercourses. Thryothorus wrens tend to live more at forest edge than in deep unbroken forest, doubtless owing to the presence of denser undergrowth in areas exposed to sunlight. Consequently, several members of the genus, such as the Plain Wren (Thryothorus modestus), have thrived in somewhat disturbed habitat where climax forest has been partially cut over or allowed to regenerate. Sometimes, there is an apparent division of habitat utilization between two species. In Colombia, for example, the Bay Wren tends to occur at forest edge, being replaced in the deeper interior by the Sooty-headed Wren (Thryothorus spadix), while in western Mexico a frequently sympatric pair of species, the Happy and Sinaloa Wrens (Thryothorus sinaloa), may have slightly different requirements, the latter being able to tolerate sparser woodland. Some species are largely associated with specific vegetation; among these are the Plain-tailed (Thryothorus euophrys) and Inca Wrens (Thryothorus eisenmanni), which usually occur in areas with Chusquea bamboo. Typically, the Plain-tailed Wren is the first colonist of highly disturbed land, such as landslips in the precipitous Andes, a habitat which is mimicked by spoil slopes from road construction. In Bolivia, Moustached Wrens (Thryothorus genibarbis) are often found in Bambusa bamboo, whereas Heliconia thickets in Costa Rica are the preferred habitat of both the Bay Wren and the Black-bellied Wren (Thryothorus fasciatoventris). Because of their preference for secondary growth, many species in this genus have survived, or even prospered, with man-made habitat change. The Stripe-breasted Wren (Thryothorus thoracicus) in Costa Rica has adapted well to cacao plantations at low altitudes and to shade coffee plantations at higher elevations.
The only temperate-zone Thryothorus, the Carolina Wren, which, according to recent studies, may not really belong in this genus (see Systematics), lives largely in highly modified habitats, such as abandoned farmland and well-vegetated suburbs. Its natural habitats included oak (Quercus) hardwoods and mixed woodlands of oak and pine (Pinus) having a variety of other tree species, such as hickory (Carya) and cottonwood (Populus). The two insular subspecies of the Carolina Wren, burleighi and nesophilus, are found in slash pine (Pinus elliottii) and palmetto stands.
In contrast to Thryothorus, the highly terrestrial members of the genus Microcerculus are much more restricted to undisturbed virgin lowland humid forest, and do not take at all well to habitat modification. The opening-up of the forest canopy induces lower-level growth unsuitable for these wrens. The four Microcerculus species are generally found at lower altitude, usually from sea-level to 1500 m, although the Flutist Wren (Microcerculus ustulatus) is a more montane species, being encountered at up to 2100 m on tepui summits in Venezuela.
Four troglodytids are closely associated with rocks. Ironically, the American Rock Wren exhibits the least morphological adaptation to this habitat, not differing greatly, in external form, from members of the genus Thryothorus. It occurs in habitats that range from fairly dry to very arid, and is usually to be seen in barren rocky places, including scree slopes, boulder falls and areas of bare, sunbaked mud. It will take advantage of mining and quarrying activities, occurring in areas of spoil and tailings, as well as colonizing areas temporarily made bare by clear-cut forestry. As a vagrant, with records over much of eastern North America, the American Rock Wren is often found in artificial situations which resemble its preferred habitat, such as concrete structures.
The three other rock-dwelling species are truly petrophilous, being associated with rock faces and showing major morphological adaptations to exploit this niche. Canyon Wrens are, indeed, associated exclusively with canyons, rock faces and, in California, sea-cliffs. Given this requirement, this species is quite catholic in its habitat choice, occurring in cool uplands at up to 3000 m, in dry oak-wood canyons or, in southern Mexico, in humid areas as low as 200 m. In contrast, the two members of the genus Hylorchilus are very specific in their habitat requirements and, as a consequence, have highly restricted geographical ranges. Both Sumichrast's and Nava's Wrens are found solely in forested karst limestone outcrops. The former can tolerate some slight changes to the forest cover, including the planting of shade coffee, since this does not impinge on the rock faces where it feeds, but for Nava's Wren any disturbance seems to be unacceptable.
Some wrens have radiated into marshland and swamp, exploiting niches that in the Old World are occupied mainly by various warbler genera of the family Sylviidae. The four species of Cistothorus have all specialized in wet, mostly monocotyledonous habitat of varying types. The Marsh Wren, as a breeding species, is usually confined to vegetation over standing waters, normally cat-tails (Typha), bulrushes (Scirpus) and reeds (Phragmites) in northern freshwater marshes, and cordgrass (Spartina) and sedges (Juncus) in brackish coastal marshes. Northern interior populations spend the winter in other fresh and brackish marshes, in the latter case frequently co-existing with resident, non-migrating individuals of the species. The Sedge Wren, on the other hand, in North America, usually avoids areas of standing water, preferring damp sedgy meadows, often with small bushes, although it may occur also in weedy rice paddies in the southern United States. In its extensive South American range, however, this wren's habitat varies enormously, from cold páramo in the Andes to open grassy savanna in Venezuela, dry cerrado grassland in Brazil, wet alder (Alnus) forest and bamboo bogs in Colombia, the borders of tidal marshes in Argentina and tussac grass in the Falklands. The great diversity of habitat occupancy, along with vocal differences, gives support to the argument for the separation of South American forms taxonomically from those in North and Central America (see Systematics).
The two subspecies of Apolinar's Wren differ markedly in their habitat requirements. The nominate race lives in similar lake-edge habitat to that occupied by the Marsh Wren, namely reedbeds dominated by cat-tails and bulrushes, albeit at altitudes of 2500 m to slightly over 3000 m. In contrast, the race hernandezi occurs higher up, between 3800 m and 3900 m, in boggy páramo with low shrubs and a hirsute leafy ground plant known locally as "frailejón" (Espeletia grandiflora).
Restricted to a very small area of Cuba, and one possessing a unique habitat, the enigmatic Zapata Wren has very specific habitat requirements. These are savanna-type swamp with sawgrass (Cladium jamaicense) and rushes, and with some bushes which can be used as songposts. The water level in the Zapata Swamp is quite variable, but the Zapata Wren tends to live in drier areas, which allow it to forage on the ground.
Another wetland-dwelling species currently included within the family is the Donacobius. This relatively large bird, of uncertain taxonomic affiliation (see Systematics), inhabits brushy vegetation along riversides and the overgrown margins of water impoundments in the lowlands of tropical South America. Although it sometimes forages away from such areas, it invariably nests only in marshy vegetation.
In North America, the Northern Wren occurs in such boreal-type habitats as moist coniferous forest with extensive underbrush, but also in mixed hardwood-conifer stands. In addition, it is also found, in an extensive range of subspecies, on treeless or nearly treeless oceanic islands in the Aleutian chain. The same species, having invaded the Old World via the Bering bridge, has, in the absence of competition from other troglodytids, expanded into a catholic range of habitats in three continents. In western Europe, the most favourable habitat is damp woodland, either deciduous or mixed, with extensive undergrowth, but the species is also common in suburban gardens, in moorland scrub and on oceanic islands with scanty vegetation. In other parts of its Old World range it may be more restricted, as, for example, in the Atlas Mountains of Morocco, where it is largely confined to stream valleys and forests at altitudes between 1200 m and 1800 m; in Nepal, it is found at elevations of 2500 m to 4800 m.
In the Americas, the "house wren superspecies" (see Systematics) occupies many of the ecological niches exploited by the Northern Wren in the Palearctic Region, especially highly modified landscapes such as wood edges, well-vegetated suburban areas and regenerating abandoned farmland. The Southern House Wren, being unable to tolerate unbroken forest, does not occur over much of Amazonia, but it rapidly colonizes suitable clearings when they appear. Over the remainder of the continent, this species occupies a remarkable range of habitats, from quite highly urbanized situations to montane areas and arid regions with dry bushland, from sea-level up to 4000 m. In the Falkland Islands, the isolated Cobb's Wren (Troglodytes cobbi), obviously closely related to the mainland house wrens, has adapted to a harsh environment consisting of moorland and the tussac grass Paradiochloa; unlike any other troglodytid species, it has learnt to forage among seaweed on tidal shores, and it does in fact occur at higher densities in places where territories actually abut the sea. In contrast to the house wrens, the group of montane Troglodytes wrens occurring from southern Mexico southwards to northern Argentina is much more specific in its habitat requirements. This group, comprising the Mountain, Santa Marta (Troglodytes monticola), Ochraceous, Rufous-browed and Tepui Wrens, is usually restricted to various forest types, always in uplands and often at considerable altitude, in the case of the Santa Marta Wren up to 4800 m. Similar habitat, that of bamboo thickets in páramo and sub-páramo, is required by the appropriately named Timberline Wren.
Bewick's Wren often co-exists with, and perhaps suffers from competition from, the Northern House Wren in eastern North America. Here, it has benefited from European colonization, as semi-open habitat was created by the clearing of climax forest for farmland. In western North America, it occupies a very wide variety of habitats, including chaparral brushland, cactus-dominated scrub and riparian woodland, some of which are unsuitable for house wrens.
Forming a group of four closely related species, the wood-wrens are found in forested land from Mexico south to Bolivia. Notwithstanding their great morphological and behavioural similarities, they have developed distinct habitat requirements and only rarely occur together. The White-breasted Wood-wren (Henicorhina leucosticta) inhabits wet lowland broadleaf forest, mostly below 1500 m, whereas the Grey-breasted Wood-wren lives in humid mountain forest, usually above 1500 m, and extending up to páramo edge at 3800 m in Costa Rica. The Bar-winged Wood-wren (Henicorhina leucoptera) is found very specifically in a few high-altitude forests, usually very impoverished, with stunted trees and heavy ericaceous undergrowth, on leached quartzite soil, a habitat which it sometimes shares with the previous species. The fourth species, only recently distinguished as a separate species from the Grey-breasted Wood-wren (see Systematics), is the Munchique Wood-wren, which has hitherto been recorded only at 2250-2640 m in extremely wet, stunted cloudforest on the Pacific slope of the Colombian Andes. It is ecologically sharply separated from the two neighbouring subspecies of the Grey-breasted Wood-wren, one of which lives at lower elevations on the same slope, while the other occurs on the drier eastern slopes of the Munchique massif. Although all three taxa can be seen within a distance of 1 km in some areas, no two of them have ever been found together.
The genus Cyphorhinus contains three species which, in appearance and habits, seem to be evolving convergently with the terrestrial antbirds, the habitat of which they frequently share. All occur in wet forest with extensive undergrowth. One, the Chestnut-breasted Wren (Cyphorhinus thoracicus), is a montane form, found in wet upland forest and cloudforest, often with a luxuriant growth of mosses and epiphytes. The other two, the Song Wren (Cyphorhinus phaeocephalus) and the Musician Wren (Cyphorhinus aradus), are lowland species, inhabiting humid forest and second growth with an extensive understorey, or várzea, from sea-level to 1000 m.
In the Andes, from Colombia southwards to northern Bolivia, the four species in the genus Cinnycerthia are very similar to one another not only in appearance but also in habitat requirements. All are found in wet mountain forest with a luxuriant understorey, usually between 1500 m and 3500 m, and often in association with thickets of Chusquea bamboo or in areas with extensive wet mosses growing epiphytically on tree boles and low branches. This genus as a whole does not tolerate extensive habitat modification.
The two Odontorchilus species are unusual among wrens in that they feed mostly in the forest canopy. As a consequence, both are restricted to areas of suitable forest, usually those which have not undergone much human modification. The Tooth-billed Wren (Odontorchilus cinereus) is a lowland species, found in tropical Amazonian forest below 500 m, while the Grey-mantled Wren (Odontorchilus branickii) lives in humid montane upper-tropical and subtropical forest of the Andes from Colombia south to Bolivia. The latter lives at 1400-2200 m on the Amazonian drainage, but can be observed as low down as 800 m on the Pacific slope.
The large wrens in the genus Campylorhynchus have succeeded in colonizing some habitats which are denied to smaller members of the family. No other troglodytid genus has been so successful in arid areas of Mexico and the south-western United States, although several other small insectivorous birds from different families, such as the Verdin and the gnatcatchers, have managed to exploit very arid landscapes. The Cactus Wren (Campylorhynchus brunneicapillus) occurs in a number of different types of desert, although always with the proviso that there be spiny cacti, such as prickly-pear (Opuntia) or various species of cholla (Opuntia), in which to nest. In the absence of such cacti, as, for example, in uniform stands of mesquite bushes (Prosopis), the Cactus Wren is generally absent. Cactus Wrens adapt quite well, albeit at a lower density, to highly degraded and disturbed habitats, including such areas as gravel pits and junkyards, so long as sufficient suitable vegetation remains for foraging and nesting.
Several other members of the genus, such as the Fasciated (Campylorhynchus fasciatus) and Boucard's Wrens (Campylorhynchus jocosus), also specialize in semi-arid habitats, although none has been able to exploit true desert to the same degree as has the Cactus Wren. A number of species, such as the Grey-barred Wren (Campylorhynchus megalopterus), are found at higher altitudes in Middle America, when their habitats include pine and pine-oak forest. The Rufous-naped Wren (Campylorhynchus rufinucha) occurs mostly on the drier Pacific slope, where it is frequently associated with bull's-horn acacias (Acacia collinsii and A. cornigera). These trees are host to aggressive symbiotic ants of the genus Pseudomyrmex, which provide the nesting wrens with vicarious protection from predators such as white-faced capuchin monkeys (Cebus capucinus) (see Breeding). In other parts of Middle and South America, species of Campylorhynchus have adapted to much more humid forest, where they may specialize in foraging in epiphytes, plant forms that are generally absent in arid areas. Troglodytids occurring in such humid biotopes include the White-headed Wren (Campylorhynchus albobrunneus) and the Band-backed Wren, although, in areas where the two are sympatric, the former tends to be found in the wetter habitats. The Band-backed Wren is, in fact, a very adaptable species, occurring in a wider range of habitats than does any other member of the genus; it inhabits humid forest at sea-level in Mexico and Guatemala, palm groves, wet epiphytic and drier pure oak forest at intermediate altitude and, at 3000 m, cypress (Cupressus) forest. It can also tolerate substantially disturbed habitat, such as forest clearings and clumps of trees bordering farmland.
Among the different groups of wrens there is a considerable diversity of habits. The great majority of the species are found in forest of various types or at forest edges, mostly in tropical areas. These are complex biotopes, with several different ecological niches, the exploitation of which by different troglodytid genera has given rise to quite diverse behavioural patterns. Several groups of wrens keep on or close to the ground. The most highly terrestrial are the Microcerculus nightingale-wrens, which spend most of their time on the forest floor, walking with a mincing gait, constantly teetering the tail in the manner of an Ovenbird (Seiurus aurocapilla). Nightingale-wrens fly only infrequently, and they are rarely caught in mist-nets, as they simply walk underneath them. In both behaviour and external appearance they are obviously converging evolutionarily with a group of small formicariids, the antpittas, the habitat of which they share. Almost equally terrestrial are the Cyphorinus wrens, which forage on or near the forest floor and which, again, share many morphological characteristics with the unrelated terrestrial antbirds.
Wrens enjoy a reputation for furtiveness. Indeed, the majority of species are relatively difficult to observe, as they readily disappear into dense vegetation at the slightest disturbance. The Microcerculus wrens have developed this behaviour to a fine art and can be extraordinarily difficult for even the most patient watcher to see. By contrast, the large wrens of the genus Campylorhynchus are raucous, boisterous and extrovert birds, frequently singing from exposed posts, in suburban areas even from the tops of lampposts, and generally acting in a rambunctious and uninhibited fashion, travelling in noisy groups that are very easy to locate. In terms of general behaviour, however, most members of the family fall somewhere in between these two extremes. Species such as Sumichrast's Wren can be very secretive, but frequently are overcome by a sense of curiosity that impels them to emerge in order to sneak a look at the intruder. Without their loud and persistent songs, many wren species, especially those inhabiting dense tropical forest, would go largely undetected.
The wrens living in temperate climates tend to be solitary or to occur in pairs, the young associating with their parents for only a limited period of time. Among tropical genera the situation is more variable. The genus Cinnycerthia is notably gregarious. The Rufous Wren (Cinnycerthia unirufa), for example, occurs in parties of up to 20 individuals, more than can be accounted for by an extended family, and frequently joins mixed flocks with other species. Several other tropical wrens occur in mixed flocks, as typified by the two Odontorchilus species, the Tooth-billed and Grey-mantled Wrens, which frequently associate with mixed parties of tanagers (Thraupidae), becards (Pachyramphus) and other canopy-dwellers. Similarly, the Stripe-throated Wren (Thryothorus leucopogon) routinely occurs in mixed flocks of antwrens (Thamnophilidae) and other species, whereas several of its congeners, such as the Plain-tailed Wren, apparently never do so. Some species routinely follow swarms of army ants (see Food and Feeding). For species with co-operative nesting strategies, notably the tropical members of the genus Campylorhynchus, groups of blood-relatives associate and act collaboratively to defend the nest and rear the young (see Breeding).
A few troglodytids, such as the Marsh Wren, will sing at night, especially in moonlight, but generally wrens are strictly diurnal. Most members of the family roost in cavities or in nests. Multiple nest-building during the breeding season is common among wrens, in some cases, as in the Marsh Wren, to an extraordinary degree, leaving numerous surplus nests for roosting. Some species, however, build nests for specific use as roosting sites. These include many of the Campylorhynchus wrens. Cactus Wrens, for instance, build nests throughout the year, and the juveniles may make tentative efforts at nest-building within days of fledging, with serious construction by ten weeks, obviously long before there is any chance of the youngsters themselves breeding. Wood-wrens, too, construct special roosting nests. In the case of the lowland-dwelling White-breasted Wood-wren, these are quite different from the breeding nest, being of very flimsy construction, and often with little effort at concealment; they are usually placed higher up in forks of slender saplings, so that a predator climbing to the nest cannot avoid shaking the support and alerting the occupant. In contrast, the Grey-breasted Wood-wren, living in the colder highlands, builds a thick-walled, well-insulated and snug roosting nest. White-bellied Wrens (Uropsila leucogastra) also build two types of nest, one a flimsy, thin-walled construction that is presumably intended for roosting only.
Many wrens roost as family groups, or the female sleeps with the young. Juveniles may then roost alone, sometimes, as with the Cactus Wren, in nests not of their own construction. Communal roosting also occurs among Northern Wrens. In this case, the birds forage as individuals during the daytime, gathering together only at dusk, the incentive being mutual body-heat conservation during long, cold nights. Northern Wrens frequently roost in cavities, usually with up to ten individuals together, but up to 96 have been found in one site in Europe and 31 in North America. In roosting aggregations of this kind, the wrens position themselves in layers, and the individuals in each layer hold the head pointing inwards, so that a rough circle is formed.
One extraordinary type of behaviour, found among many wrens of different genera and in totally different habitats, is the destruction of the eggs and, sometimes, the young of other birds, sometimes of conspecifics but very frequently those of other species. Both the Giant Wren of southern Mexico and the Bicoloured Wren (Campylorhynchus griseus) of northern South America have acquired the local dialectal name of "chupahuevo", meaning literally egg-sucker, apparently owing to a propensity of these wrens to enter chicken coops and attack the eggs therein.
Other troglodytids have developed egg destruction to a much greater degree than an occasional raid on a hen-house. In temperate North America, the Northern House Wren can be extremely destructive of other species. House wrens will enter the nests of other cavity-using species, such as the Carolina Chickadee (Poecile carolinensis), the Tufted Titmouse (Baeolophus bicolor) or the White-breasted Nuthatch (Sitta carolinensis), and either pierce the eggs or throw them out. Not all of the wren's aggression, however, is directed at cavity-nesters; species such as the Chipping Sparrow (Spizella passerina) may also be victimized, and quail (Odontophoridae) eggs experimentally placed in open nests in Northern House Wren territories may be attacked. The destructive effect of this troglodytid on other avian species has been observed for more than a century, and gave rise to a superbly vitriolic paper by a certain A. Sherman, published in the Wilson Bulletin of 1925, entitled "Down with the House Wren boxes". Sherman documented in great detail the crimes of the "felonious House Wren", prophesying that "if no steps are taken to stop his unrestricted breeding it is safe to predict the time will come when all true bird lovers will wring his neck as cheerfully as they now wring the neck of the pestiferous English Sparrow". Her solution was that persons who put up nestboxes used by these wrens should take them down again, so that "a kind Providence need not protect his memory from the just execrations of future generations". In one interesting instance observed by Sherman, a Northern House Wren entered a building where there was a preserved shrike (Lanius) nest with blown eggs; the wren pitched the eggs out on to the floor. On being replaced, the eggs were again tossed out by the wren on a second raid, by which time they were a little the worse for wear.
There is circumstantial evidence that nest predation by house wrens may have been heavily implicated in the decline of Bewick's Wren in eastern North America. In most areas, the appearance of the one coincided with the disappearance of the other. Certainly, the Northern House Wren has a substantial impact on the Prothonotary Warbler (Protonotaria citrea) in Canada, where this cavity-nesting parulid is considered endangered (see Status and Conservation). In southern Ontario, nest predation by the wrens is one of the major causes of nesting failure among the warblers.
Although egg predation by Northern House Wrens has attracted the most attention by virtue of the species' close association with humans, other, non-cavity-nesting troglodytids, notably the marsh wrens in the genus Cistothorus, are also well known as predators of eggs. In eastern North America, the Marsh Wren frequently attacks the eggs of Red-winged Blackbirds (Agelaius phoeniceus), as well as those of species up to the size of the Least Bittern (Ixobrychus exilis). Intraspecific egg destruction also occurs; females have been seen to kill the chicks of wrens in adjacent territories and to throw them out of the nests. Marsh Wrens can even be caught in traps baited with small eggs. Furthermore, incubating females of the Marsh Wren may show considerable hostility towards their mates, doubtless in order to protect their own eggs; these are unusually thick-shelled, perhaps as a defence against attack. Marsh Wren predation may have a significant negative effect on the breeding success of Red-winged Blackbirds, which, for their part, are very aggressive towards wrens. In western North America, a similar state of mutual hostility exists between Marsh Wrens and Yellow-headed Blackbirds (Xanthocephalus xanthocephalus); the blackbirds may destroy wren nests in order to prevent a brooding female from returning to her eggs. In Colombia, Apolinar's Wren appears to have an equivalent mutually hostile relationship with the local icterid, the Yellow-hooded Blackbird (Agelaius icterocephalus).
The Sedge Wren in North America exhibits the same behaviour towards marsh-dwelling icterids and may have a significant adverse effect on their populations. Sedge Wrens have been seen to attack the eggs of species as large as the Cinnamon Teal (Anas cyanoptera), but without being able to penetrate the shell. Several other troglodytids, such as the Plain Wren in the genus Thryothorus, have been seen to prey on other species' eggs or young, and additional observation and fieldwork would doubtless reveal that this behaviour is quite general.
A number of explanations have been advanced for the phenomenon of egg predation by wrens. In some cases the eggs may be wholly or partially eaten, but frequently they are not; eggs are not, therefore, a significant food source. Intraspecific egg destruction may be a useful way of reducing competition for food resources at the period of maximum demand, and it may be relevant that Marsh Wrens, for example, sometimes nest at very high densities with tiny territories. This explanation, however, is less plausible when applied to interspecific predation involving large unrelated species such as the Yellow-headed Blackbird. In the case of the cavity-nesting house wren, the act of destroying the nests of other cavity-nesting species may, in the long run, ensure a supply of nesting sites for the wrens; the latter, being migratory, arrive and commence nesting later than do some resident victims such as the Carolina Chickadee or the Tufted Titmouse. Perhaps significantly, the non-cavity-nesting Northern Wren appears not to prey on eggs. On the other hand, the highly predatory marsh wrens are also non-cavity-nesters.
The behaviour and general habits of the Donacobius resemble those of the mockingbirds more than they do those of the wrens, a fact that, prior to recent DNA investigations, was used by some authors to argue that this species belongs in the family Mimidae (see Systematics). This is a noisy, extrovert bird, usually occurring in small groups of up to four, blood-related individuals. Pairs perform ritualized mutual displays, with much flirting and wagging of the long tail, which is held such that the conspicuous white tips are on show. A more intense form of display involves the adoption of a hunched-back posture, with the head and tail held down, the throat grossly distended, and sometimes with the wings drooped to reveal the conspicuous white flash on the primaries. Both sexes sing. During mutual song sessions, the pair-members may sit close to one another, waving the long tail rhythmically from side to side, with the inflatable yellow throat patch distended. As the Donacobius seems to form long-term pair-bonds, breeds co-operatively and has a rather extended breeding season, it is somewhat difficult, if not impossible, to distinguish between possible breeding displays and general social activity.
For many troglodytids living in arid conditions, water-bathing is not an option. Indeed, even when water is available, the American Rock Wren and the Canyon Wren have not been observed to indulge in this activity, preferring instead to dust-bathe. Cactus Wrens, too, habitually dust-bathe, but they will bathe in water when it is present, even visiting suburban birdbaths for that purpose. Marsh Wrens, but not Sedge Wrens, have been seen to bathe in swamp water. Curiously, water-bathing has not been reported for Northern House Wrens, although both this species and the Carolina Wren do dust-bathe. Bewick's Wren bathes in both water and dust. There are a few reports of sun-bathing. Carolina Wrens have been seen to lie in the sun with the eyes half-closed, the wings and tail partially spread, and the feathers of the head, back and rump raised.
Most species of wren have been seen to preen, head-scratching being of the indirect type, with the foot lifted over the wing. Allopreening is apparently rare among troglodytids, although it has been observed for a few species.
Aggressive behaviour by wrens is generally associated with territoriality and breeding. Threat displays usually involve the fanning of the wings and the drooping and partial spreading of the tail, while pointing the bill at the object of aggression. Actual physical combat, however, is uncommon. Typically, two individuals will fly vertically upwards for a short distance, while flapping the wings into each other, grasping each other's feet and sometimes pecking. Occasionally, such efforts are directed against other species. American Rock Wrens, for example, have been seen to attack Northern House Wrens which were attempting to pilfer nesting material from their nest, and have also been observed to attack the much larger Western Kingbird (Tyrannus verticalis).
The Troglodytidae include some of the finest and most appreciated singers among all birds. The Northern Wren, for example, heard throughout the Holarctic Region, is appreciated for the long duration of its song and for the degree of expressiveness contained within its singing. This species' songs range from barely audible whispered ones, given while courting a female, to songs delivered with a truly surprising vehemence for so tiny a bird. It is the songs of wrens that define certain habitats throughout much of the New World. Examples are the cascading, sweet liquid whistles of the aptly named Canyon Wren, heard throughout canyons in western North America, the rhythmic "char char char char" of the Cactus Wren in the deserts of the south-west United States and northern Mexico, the unmistakable gurgling of Marsh Wrens, inseparably linked with cat-tail and bulrush marshes from the Pacific seaboard across to the Atlantic coast, the "tea-kettle tea-kettle tea-kettle" song of the Carolina Wren in the Deep South of the USA, and similar striking songs and duets of other Thryothorus wrens throughout Central and South America.
Although most wrens bear English names that helpfully allude to their distinctive plumages, or to the habitat or geographical area in which they are found, some have escaped such mundane appellations and have been given names drawing attention more to their vocal abilities. Thus, two of the Microcerculus species are known as nightingale-wrens and another as the Flutist Wren, while the Central American subspecies luscinia of the Southern Nightingale-wren (Microcerculus marginatus) is generally referred to as the Whistling Wren; and two of the three Cyphorhinus species are the Song Wren and the Musician Wren. Given the appealing qualities of these songsters, it is no surprise that they have attracted considerable attention, both aesthetically and scientifically.
Most members of the family achieve their remarkable songs by learning, the young birds imitating the songs of adults in the population. This was nicely demonstrated in a study of Bewick's Wrens undertaken by Kroodsma in the north-west United States. In the Willamette Valley of Oregon, where Bewick's Wrens are resident, a young male stays with his parents until four or five weeks of age, and then sets out to find a territory of his own. By eight weeks of age, he will probably already be on his own territory, one that he will retain for the rest of his life. There, the youngster begins to practise singing; his first attempts are only scratchy, continuous jingles, but his efforts gradually improve, and within a few weeks he is already singing what are clearly attempts at imitations of the songs of adult males around him. The young male perfects his songs during the autumn and over the winter, and by the following spring has acquired the songs of his immediate neighbourhood, as those are the ones that he must use in order to function there. Although he was capable of imitating his father's songs before dispersing, all of these were eventually rejected and replaced with the songs that he needed at his permanent breeding location.
This learning process is easily demonstrated also in laboratory experiments, as with Marsh Wrens. If a ten-day-old Marsh Wren is removed from the nest and hears no songs over the next year, he sings "nonsense" songs, songs that are unrecognizable as to species. If he is allowed to hear some Marsh Wren songs over loudspeakers, however, he attends to the minutest of details in those songs, imitating them with remarkable precision. In the laboratory, males can begin to imitate songs broadcast from loudspeakers when they are as young as 15 days of age, with a peak of learning ability when between 25 and 40 days, and perhaps a secondary peak ending around day 60; little to no learning of additional songs occurs after 75 days. More realistic learning experiences can be achieved by allowing a juvenile to learn songs from an adult, singing male, and the sensitive phase for learning can then be extended well beyond 75 days, to the following spring, at a time when a migratory yearling male would establish his first breeding territory. Flexibility in where and when to learn is necessary for these wrens, as some nestlings hatch in the period after the adults have stopped singing for the year, and those nestlings must delay much of their learning until the following spring.
Most wrens probably learn their songs in this fashion, whereby young birds disperse to a breeding location and learn their songs there. One of the best forms of evidence for this is vocal dialects, in which the songs of the wrens vary over short geographical distances. Songs of Bewick's Wrens, for example, change gradually over distance, and sometimes change abruptly at discontinuities in habitat, so that the songs in a father's neighbourhood are different from those in the neighbourhood where his young male offspring establishes a territory. Songs of Marsh Wrens, and the sequences in which individuals use them, vary from one marsh to another, as J. Verner found in the 1970s. Similarly, Kroodsma recorded that songs of American Rock Wrens at a site in eastern Oregon were different from those uttered by the species at another locality only 160 km away. In Europe, the songs of Northern Wrens can differ over very short distances, a small lake being enough of a barrier that songs on one side are different from those on the other. Likewise, among Carolina Wrens in the United States, it was found that songs changed rapidly over a 145-km transect; most versions were heard at only a single locality, and most songs in the wren repertoires changed abruptly at dispersal barriers.
The one known exception to this process of precise song imitation and the formation of song dialects involves the North American population of the Sedge Wren, which is closely related to, yet so different from, the Marsh Wren. A nestling Sedge Wren, when taken into the laboratory and exposed to songs of adults of its species, imitates none of those songs. Instead of copying songs from the loudspeakers or from other young birds nearby, he improvises a sizeable repertoire of unique songs, such that few, if any, of the songs that he perfects are like those of others of the species. Consistent with this style of song development is the evidence from wild-living individuals in North America, where neighbouring males also have unique repertoires of songs. In effect, Sedge Wren songs are like snowflakes, as no two seem to be exactly alike, but each is instantly recognizable as being the song of a Sedge Wren because all songs are built on the same principle, a few introductory notes followed by a dry, staccato trill, "cut-cut-cut-trrrrrrrrrrrrrr".
Why the North American Sedge Wren seems to differ from all other troglodytids in its style of song development may perhaps be explained by its unique reproductive biology. Individuals that are ringed at one location early in the season, in the northernmost part of the range, typically leave there during mid-season, presumably going elsewhere to breed; as these wrens often arrive to breed at lower latitudes, such as Nebraska, Indiana and Kentucky, during July and August, it appears that they make a stopover to nest on their migratory route back to the coast of the Gulf of Mexico. These Sedge Wrens are rather nomadic, and not at all site-faithful. They rarely return to the same breeding location in successive years; in one unpublished study, S. Johnson found that, of 300 individuals, only one returned to breed at the site where it had nested in the preceding year.
The uniqueness of these North American Sedge Wrens is highlighted by a brief survey of other Cistothorus wrens, based largely on the investigations of Kroodsma and colleagues. In Costa Rica and the Falkland Islands, other populations of the Sedge Wren are sedentary, and neighbouring males imitate songs from one another, creating song dialects over relatively short distances. In Brazil, neighbouring male Sedge Wrens answer each other with identical songs, such behaviour being possible only if the birds learn their songs from each other. Similarly, the Merida Wren (Cistothorus meridae) of the Venezuelan Andes and the globally threatened Apolinar's Wren of Colombia are sedentary and, again, neighbouring males learn each other's songs, as evidenced by the readily documented song dialects of these species. Interestingly, the recently described subspecies hernandezi of Apolinar's Wren not only has a more varied repertoire of song types than the nominate form, but also lives in small groups in which several individuals, of both sexes, sing communally.. In North America, too, populations of the Marsh Wren are typically sedentary or, if migratory, individuals are highly site-faithful, returning annually to the same marsh.
It is likely that the unparalleled movement patterns among the North American Sedge Wrens have co-evolved with a reduced emphasis on song imitation. Local communities of birds are highly ephemeral, with individuals staying together just long enough to raise one family. They then move on to the next breeding location, either in the same year or in the following year. There is little time or opportunity to relearn large vocal repertoires at each breeding location; indeed, as the repertoire of a single male can number up to 300 different songs, it is perhaps impossible to accomplish such a feat twice in each breeding season. The solution for these Sedge Wrens is to improvise or invent songs, using some shared rules for generating species-typical songs. The result is that, no matter what the geographical origin of each singing individual, and no matter where he has been on his travels, he will instantly be recognized, both by other males and by females, as a Sedge Wren.
Geographical patterns in songs can be the first clues to the evolutionary history of the wrens that sing them. In Costa Rica, for example, F. G. Stiles discovered a sharp transition zone in the songs of nightingale-wrens, leading to the formal recognition of two species, the Northern Nightingale-wren (Microcerculus philomela) and the Southern Nightingale-wren. Farther north, in Mexico, the songs of two allopatric Hylorchilus populations were found by H. Gómez de Silva to be sufficiently distinct to warrant species status for both, as Sumichrast's Wren and Nava's Wren (see Systematics). In the Neotropics, likewise, careful listening to wood-wrens in Colombia led S. L. Hilty to suggest that the birds living at especially high altitudes represented a new species, as their songs were different from those of the wood-wrens throughout the rest of Colombia. In 2003, this high-elevation form was officially recognized as a new species, the Munchique Wood-wren (see Systematics). The songs of North American Sedge Wrens vary little, if at all, geographically, revealing what must be one large evolutionary unit in North America. In contrast, local dialects conceal these evolutionary groups to some extent in Central and South America, but major geographical differences in songs, from Costa Rica to Brazil and to the Falkland Islands, surely represent different evolutionary groups, which are perhaps, as suggested by M. T. Traylor in 1988, worthy of species status.
Another example in North America is that of the Marsh Wren populations, which almost certainly consist of two species (see Systematics), not one as currently recognized. From the Atlantic coast east to the central Great Plains one style of singer is found, and from the central Great Plains west to the Pacific another kind, and the differences between the two in behaviour are striking. The songs of western males are highly diverse, with harsh and grating sounds contrasting with loud tonal notes; those of eastern males are more liquid, without the harshness, and far less diverse. Western males introduce their songs with short, sharp "click" notes, whereas eastern males begin with a nasal buzz. In addition, western males have much larger song repertoires, containing an average of perhaps 150 different songs, far more than the 30 to 50 songs in the repertoires of eastern males, and western males race through their repertoires much faster than do eastern males. These differences in vocal repertoires are reflected in the song control centres of the forebrain, too, with western males devoting about 50% more brain space to controlling their much larger song repertoires. The extra brain space in western birds is not generated because the birds learn more songs, but, rather, it seems to be part of a genetic programme that generates sufficient brain space so that large repertoires can be learned.
Furthermore, many of these behavioural and neural differences are based on genetic differences between eastern and western birds, as nestling males taken from eastern and western populations, respectively, behave differently when they experience the same laboratory environment. Juveniles from western North America still imitate about three times as many songs as do eastern ones, and the western individuals also allocate more brain space for controlling those larger repertoires. In the laboratory, western males also race through their repertoires faster than do the eastern males. The western males are simply more proficient at learning the larger repertoires than are the eastern birds, and these differences in ability are due to genetic differences that have evolved in the two respective groups since they diverged from their common ancestor. One possible explanation for the escalation in vocal behaviour among western Marsh Wrens is that territories in the west are often smaller, so that competing males are singing more closely together, and males are also more polygynous, so that the stakes in the singing game are higher. With these smaller territories, a small marsh reaches lek-like proportions, and prospecting females can choose from among all of the competing males, a situation that may have promoted the evolution of especially large repertoires and complex singing behaviour.
Just how complex these aspects of vocal behaviour are is revealed by Verner's study of western Marsh Wrens. At Turnbull National Wildlife Refuge, in eastern Washington state, he found that neighbouring males have essentially the same repertoires of about 115 different songs, but they have learnt not only the songs, but also the sequences in which to use them. As the wrens countersing from neighbouring territories, they often answer each other with identical songs, not just once, but many times in succession; there is frequently a community effect, too, as other males within earshot are often heard to echo the same song. The possibilities for interactions are intriguing, as a male can respond to a neighbour with the song that he just sang, or jump ahead one song in the standard sequence, or give a song that is well removed from that particular sequence. The response choice that a male makes, or perhaps is allowed to make, may well signal important information to other individuals listening to this exchange, information that may indicate the relative prowess of the two singers. When a tape recorder plays a standard sequence of song within a territory, for example, a male routinely jumps ahead in the sequence, so that the tape recorder "sings" a particular song type after the resident male. It is possible that, by refusing to follow the recorder, the male signals his dominance to this simulated intruder.
As already initimated (see also Systematics), the eastern and western populations of the Marsh Wren almost certainly represent two separate species, and a similar situation prevails with the Northern Wren. The songs of males of the latter species in western North America consist of brief notes delivered at breakneck speed, so that human ears hear little more than a high-pitched blur. In contrast, eastern songs are far less complex; they are delivered more slowly, contain fewer notes, and have fewer notes per unit time. Repertoire sizes, too, differ, eastern males having an average of only two relatively simple songs apiece, whereas western males have perhaps a dozen basic songs, from which countless permutations are generated. Moreover, the non-learnt call notes differ, western Northern Wrens having a sharp, high-pitched note and eastern ones a lower-pitched, richer note. Predictably, perhaps, the songs of this species in the Old World, from Britain east to Japan, are different again. In tonal quality and organization, they are most like those of the wrens from eastern North America. Repertoires in Eurasia consist typically of six or seven songs, each given in a constant, repeatable fashion, much like the singing of eastern Nearctic wrens.
As the tropical biologist A. F. Skutch pointed out in 2001, we know far more about the few "odd" wrens that have invaded the north-temperate zone than we do about the 50 or so that have remained in the New World tropics. In the tropics, the vocal behaviour of troglodytids can be very different.
One immediately noticeable difference between the wrens in the north-temperate zone and the tropical wrens is that females in the tropics sing far more. In the case of the two house wrens, for example, females in North America sing occasionally in at least one well-studied population, in Wyoming, although perhaps not in some other populations; these Northern House Wrens are polygynous. In contrast, females of the Southern House Wren, which are monogamously paired to their males, sing far more regularly throughout the tropics. Females of the related Socorro Wren (Troglodytes sissonii), confined to the island of that name off western Mexico, also sing. Among the Cistothorus wrens, females of the temperate-zone Marsh Wren and Sedge Wren do not sing, but those of the two Andean species do; female Merida Wrens emit a simple trill as their mates sing a more complex, whistled song, and female Apolinar's Wrens utter a prolonged trill that slowly rises and falls in pitch, the male then chiming in with a briefer, more complex song. In southern Mexico, Sumichrast's Wren, in the genus Hylorchilus, exhibits similar vocal differences between the sexes: the female's song is less complex than that of the male, being a dry, staccato chatter, much like that of a male Cactus Wren, whereas her mate's song is a rich series of descending whistles.
These differences between temperate and tropical representatives of a genus are especially pronounced among the Thryothorus wrens. Only one of these, the Carolina Wren, reaches North America, and the male of that species has a repertoire of 40 or so loud, rich songs; the female, in contrast, utters a simple rattle, typically in response to her mate's song during aggressive encounters with other wren pairs. In the tropics, however, female Thryothorus wrens often have songs that are just as complex as are those of the males. The songs of both sexes of the Rufous-and-white Wren (Thryothorus rufalbus), for example, consist of a series of pure whistles gradually or abruptly accelerating in tempo, with the female's song slightly higher in frequency than the male's; a male and a female often sing simultaneously, the songs overlapping in time but less so in frequency. In the Black-bellied Wren, too, male and female songs are an equally complex series of rich whistles, often overlapping in time, but that of the the female again perhaps a little higher in frequency. Most impressive among these Thryothorus wrens are those species, such as the Buff-breasted Wren (Thryothorus leucotis), in which the male and female sing such a tightly co-ordinated duet that their respective roles are indistinguishable, unless one happens to be standing between the two singers.
Careful study of one of these duetting species, the Bay Wren, has revealed how and, to some extent, why the sexes engage in such duets. R. N. Levin found that males and females duet precisely, even when they first meet, thereby dispelling the previously held notion that it would take time, in a long-term monogamous pair-bond, for such precision to be learnt. Rather, it seems that the birds follow some general rules that enable this precision, regardless of which particular song type each contributes. Further, a lone female or a lone male can maintain the territory by itself, dispelling still another idea, namely, that the duet is needed for joint territory defence. It is the female that begins each duet, the male then joining her, and it seems therefore to be the male's behaviour, not that of the female, that makes the joint singing a duet. Perhaps this is because the male can in this way announce to other males in the neighbourhood that this particular female is taken; in other words, he is employing "acoustic mate-guarding".
Equally enlightening is the way in which territorial males and females sing and respond to songs. A paired male responds strongly to both male and female song, as if territorial towards both sexes. When he becomes unpaired, he increases his singing substantially, just as do most unpaired songbird males, and he then responds more strongly to female song than to male song, suggesting that his abundant songs serve to attract a mate. A female that becomes unpaired, however, does not change her singing rate, and lack of a male song accompanying her clearly signals that she is unpaired. Whether she is unpaired or not, she responds strongly to songs of other females but not to those of males, suggesting that she uses her songs in order to defend her territory against other females.
Ongoing studies of another Thryothorus species in Costa Rica, by L. E. Molles and S. L. Vehrencamp, illustrate the diverse singing styles within this genus. In the case of the Banded Wren, males are the primary singers, and a typical repertoire consists of about 20 different songs. Females, too, sing, and their songs are of the same general form as those of the males, but of lower amplitude; a female occasionally sings jointly with her mate during a territorial dispute. As with some other troglodytids, such as the Sedge Wren and the Marsh Wren, males sing vigorously at dawn and switch to new song types more at that time than they do later in the morning. Among these wrens, the rate of singing and the frequency with which a male switches to a new song type may signal important information to listeners about his mood, but the exact rules for conveying the appropriate information are not necessarily the same for all species.
What appears to be one of the most complex duets is that of the Canebrake Wren (Thryothorus zeledoni), a species restricted to the Caribbean slope of Central America. A study of this wren was recently undertaken in north-east Costa Rica by N. I. Mann and others, who captured and individually colour-ringed 24 territory-holding adults. Blood samples were analysed to enable these to be sexed. The researchers found that the duet of this species consisted of three main components, each one distinctive in structure. The male had a repertoire of very brief rapid phrases and the female had a repertoire of different, slightly longer phrases. In addition, the male possessed a second, separate repertoire of somewhat longer, higher-frequency phrases, each containing 4-6 elements; each duet was almost invariably introduced by one of these. Thus, a typical duet would consist of the male's introductory phrase followed by a very fast and highly co-ordinated alternation of female and male phrases. In a very few instances, the male's introductory phrase, which he also used as a solo song, was omitted. The Canebrake Wren is often considered conspecific with the Plain Wren, although it differs vocally and in plumage. Interestingly, Mann and colleagues' preliminary data on the nominate race of the Plain Wren, inhabiting the Pacific slope of Costa Rica, appear to suggest that this has a song pattern similar to that of the Canebrake Wren.
Vocalizations include more than singing, of course, and the members of this family have extensive repertoires of calls as well. Partly because wren songs are often so striking, the calls of these species have been studied relatively infrequently. The most thorough study in this respect has been done with the Stripe-backed Wren (Campylorhynchus nuchalis), a co-operative breeder (see Breeding) that occurs in Venezuela and Colombia. Price found that, in co-operative family groups, young males learn their repertoires of calls from adult males, and young females learn their calls from adult females. As a result, males in the same "patriline" have nearly identical call repertoires, no matter whether the young male stays at home or disperses to another group, and all females within a "matriline" have identical repertoires, too, but the calls are different from those of the males. This form of vocal tradition thus provides clues as to both sex and kinship, and studies using playback have shown that the males can readily distinguish male relatives from non-relatives. These wrens provide a rare example of individuals being able to recognize their kin on the basis of learnt vocal cues, and these cues undoubtedly prove useful in helping them to keep track of close kin within and among family groups, thereby enabling complex social relationships among individuals in this co-operative breeder.
Food and Feeding
Arthropods make up the bulk of the food items eaten by most troglodytids. Among the different wren genera, however, there is considerable variation both in the food and in the techniques used as a means of acquiring it.
There is a substantial body of data on the diets of most of the North American wren species, as well as that of the Northern Wren in Europe, derived from examination of stomach contents of specimens. It should be borne in mind, however, that studies of stomach contents were typically undertaken in one geographical location and were often quite limited in scope; hence, published results may not be entirely representative of the diet of a species throughout its range, or even throughout the year. Broadly speaking, invertebrates of many kinds form most or all of the food of this family. For example, American Rock Wrens take such prey as locusts and grasshoppers (Orthoptera), earthworms (Oligochaeta) and grubs, and Canyon Wrens consume spiders (Araneae), beetles (Coleoptera), bugs (Hemiptera), planthoppers (Homoptera), ants (Hymenoptera), termites (Isoptera) and similar prey, ranging in length from 2 mm to 12 mm, while Bewick's Wrens feed on bugs, beetles, bees and wasps (Hymenoptera) and caterpillars. The Northern House Wren eats caterpillars, grasshoppers, beetles, leafhoppers (Cicadellidae) and spiders, among others, and the Northern Wren's diet includes such items as grasshoppers, earwigs (Dermaptera), beetles, spiders, woodlice (Isopoda) and the like. Northern House Wrens are known to favour slower-moving prey, such as spiders and bugs, rather than such insects as flies (Diptera), the capture of which would be more challenging. This species has been seen also to take calcareous matter, such as mollusc shells, and to feed such inorganic items to its young, presumably as a calcium supplement at a time when it is needed for bone growth. Larger, vertebrate prey is occasionally found in stomachs. The American Rock Wren, for example, will take young lizards. It is of interest to note, incidentally, that neither the latter species nor the Canyon Wren appears to drink, even when water is freely available.
Vegetable matter does figure to a minor extent in the diets of some of the smaller North American troglodytids. For instance, Bewick's Wren takes some vegetable matter, especially in winter, while the Northern Wren has been observed to eat berries, including elderberries (Sambucus) and blueberries (Vaccinium), as well as some seeds and even seaweed.
The large wrens of the genus Campylorhynchus have a dietary spectrum that is wider than that of smaller troglodytids. Most members of this genus eat substantial quantities of vegetable matter, such as cactus seeds. The only one which has been studied in any great depth is the Cactus Wren; in one case, 17% of the diet of this species consisted of vegetable matter. Campylorhynchus wrens also take relatively large prey, such as lizards and frogs, and the Spanish name for Boucard's Wren, "Matraca alacranera", implies that this species eats scorpions (Scorpiones). As previously mentioned (see General Habits), the Giant and Bicoloured Wrens are known in local Mexican and Colombian dialects as "chupahuevo", meaning egg-sucker, although whether they actually eat hens' eggs or merely vandalize them is not known. Bicoloured Wrens can also be quite destructive to soft fruit, such as mangoes.
For many of the Neotropical wren species, the data on prey items are very scanty and, in many cases, few relevant observations are available. Furthermore, the notes on stomach contents accompanying specimens frequently contain unhelpful and vague generalizations, such as "insects", "insect parts" and the like. As a result of the more meticulous work of A. Wetmore in Panama and of F. Haverschmidt in Suriname, the diets of most species in these two countries are known in better detail. According to the rather limited information published, the diet of the tropical Thryothorus wrens encompasses a very great variety of arthropods. These comprise many orders of insects, including lepidopteran larvae, ants, bugs, grasshoppers, cockroaches (Blattodea) and dipteran flies, as well as other groups, among them spiders, woodlice, centipedes (Chilopoda) and millipedes (Diplopoda). So far as is known, vegetable matter, usually in the form of seeds, is a very minor component in the food of these species. The sole North American member of this genus, the Carolina Wren, will also take small invertebrates, including lizards, tree-frogs and young snakes.
In its unique sawgrass habitat in western Cuba, the Zapata Wren has learnt to exploit a more varied range of food resources, including fairly large lizards of the genus Anolis, seeds, lichen, snail eggs, slugs and a wide variety of invertebrates. By contrast, the marsh-dwelling Cistothorus wrens are more purely insectivorous, although in mid-winter, with snow on the ground, the Sedge Wren in extreme southern South America does eat seeds. Snails have not been recorded as being consumed by any troglodytids with the single exception of Sumichrast's Wren, in the diet of which they may figure quite prominently.
The food of the Donacobius consists almost entirely of invertebrates, mostly gleaned from leaf surfaces, but also taken in flight. Recorded prey items of this species include beetles, hymenopterans, orthopterans, neuropterans and arachnoids.
The feeding techniques of wrens also exhibit considerable variation and adaptation to different habitats. The members of many genera spend most of their time in foraging in the lower levels of dense vegetation, where they examine leaves for prey or rummage in leaf litter. Henicorhina, Thryothorus and Troglodytes are typical examples of such unobtrusive wrens. Nevertheless, the various species have differing habits. Some, such as the Coraya and Buff-breasted Wrens, routinely feed quite high up in trees, whereas the Rufous-and-white Wren specializes in foraging in leaf litter and is rarely seen far from the ground. The only wrens which might be described as canopy feeders are the two species in the genus Odontorchilus, the Tooth-billed and Grey-mantled Wrens, which are regularly found 30 m or more above the ground in the upper levels of tropical and subtropical forest.
Some tropical species have developed a technique employed by antbirds and many other Neotropical birds, that of following swarms of army ants, opportunistically snapping up invertebrate prey flushed from otherwise secure hiding-places by the insects. Typical in this respect are the terrestrial Cyphorhinus wrens, such as the Song and Musician Wrens, which bear a superficial resemblance to some of the formicariid antbirds. Others include the Sooty-headed Wren in the genus Thryothorus and, at least where its range overlaps that of army ants, in southern Mexico, the White-bellied Wren in Uropsila. The lowland White-breasted Wood-wren is also a typical ant-follower, but its relative, the highland Grey-breasted Wood-wren, is not, as it is altitudinally separated from army ants. Not unexpectedly, the degree of association with army ants varies. Some species, such as the Riverside Wren, will join swarms briefly, before losing interest and foraging elsewhere.
Northern Wrens are somewhat unusual within the family in that they have been seen to prey on tadpoles and tiny fish, by wading into shallow water and actually immersing the head as they search for these food items. They have also been observed while attending the European badger (Meles meles), much in the manner of Cattle Egrets (Bubulcus ibis) following livestock, and catching prey disturbed by the mammal's foraging activities. The rock-dwelling wrens, the Canyon, Sumichrast's and Nava's Wrens, have developed special anatomical features, such as a long, decurved bill, a flattened skull and widely splayed legs, which enable them to forage deeply into crevices in rock faces. In addition, Canyon Wrens will often sally forth briefly from high rock walls to seize insects flying nearby. Classic flycatching behaviour, involving the mid-air pursuit of flying insects, is uncommon among the members of this family, although Northern Wrens may occasionally practise this method of foraging.
Most terrestrial wrens are very secretive, the two nightingale-wrens obsessively so, feeding only in places where they are well hidden. The Wing-banded Wren (Microcerculus bambla) appears to specialize in finding prey in rotten logs lying on the forest floor, where it vanishes for several seconds inside cavities in hollow fallen tree trunks. Campylorhynchus species, on the other hand, are anything but furtive. The Cactus Wren, for example, frequently feeds on the ground well in the open, even sometimes on suburban lawns.
Several troglodytids have developed opportunistic feeding strategies. Carolina Wrens, especially during the winter months, come to feeding stations, where they are especially fond of suet. They have also been seen to take sunflower seeds and to crack them open in the manner of a nuthatch, by wedging the seeds into a cranny and hammering them with the bill. Canyon Wrens may pilfer the paralysed spiders with which mud-dauber wasps (Scelifron cementarium) provision their nests, while the White-bellied Wren will steal prey from spiders' webs. On Guadalupe Island, off Baja California, the local subspecies guadeloupensis of the American Rock Wren, restricted to that island, has learnt to frequent slaughterhouses, where it feeds on flies attracted to the offal. Another example of opportunism concerns the Cactus Wren. In suburban situations in the American South-west, some individuals of this species have discovered that the squashed insects on the radiators of cars are a reliable source of conveniently pre-cooked protein.
Among different groups of wrens there exist a number of breeding features and strategies that are quite unusual in passerines. These include polygamy, usually in the form of polygyny but also, more rarely, polyandry, as well as co-operative breeding and an extraordinary degree of multiple nest-building. It should, however, be admitted that there are major gaps in our knowledge of the breeding biology of the Troglodytidae. Indeed, for more than a quarter of the tropical wrens, not even has the nest been described, and detailed studies of the breeding strategy exist for only a few species.
The fact that wrens build more than one nest in a single season has been well known for many years. In England, for example, many decades ago, every bird-nesting schoolboy was familiar with the phenomenon of the "cock's nests" of the Northern Wren. The degree of multiple-nesting varies greatly among wren genera, apparently not occurring at all in some, but reaching an extraordinary degree in the Cistothorus marsh wrens, and a peak in the Marsh Wren itself. Male Marsh Wrens may, in the course of a season, construct up to twenty separate nests. From among these the female will then choose a suitable one, which she then completes, or she may even, apparently, induce the male to build yet more. It takes the male perhaps two days to build a candidate nest, but it can require eight days for both sexes to complete the nest that is to be used for breeding. Given the very substantial resources demanded of the male for this construction bonanza, there must be a major payback in evolutionary terms. The precise nature of this is, however, a matter of considerable debate. Certainly, in the event of a disaster befalling a breeding nest, it is obviously advantageous to have a half-completed back-up nest at hand. In a cat-tail marsh, where it is not easy to conceal a nest, the presence of numerous "decoy" nests may confuse a predator, and it is perhaps significant that the Sedge Wren, which nests in more tangled vegetation where concealment is easier, builds only half as many surplus nests as does the Marsh Wren. It has been suggested that the sheer vigour of a male's nest-building activities may indicate to the female his breeding fitness. Surplus nests are also useful for roosting, but why would twenty such nests be required? Probably, no single hypothesis completely explains the phenomenon. The building of additional nests for roosting is widespread among the members of this family. In the case of some species, such as the White-breasted Wood-wren, the Riverside Wren and the White-bellied Wren, the dormitory nests are slighter in construction and more poorly concealed than are the breeding nests, and the roosting nest of the Plain Wren, for example, is so flimsy that the bird can make an emergency exit by bursting out through the back wall.
For a number of genera, among them Odontorchilus, Hylorchilus and Microcerculus, there is no documented evidence that multiple nests are built, although this may be due to a lack of observation. Many Thryothorus wrens build dormitory nests but not, apparently, multiple breeding nests. One species, the Carolina Wren, a debatable member of this genus (see Systematics), occasionally builds spare nests in the breeding season. By way of contrast, the constructing of multiple nests appears to be standard in the genus Troglodytes, at least for those of its members which have been adequately studied. In Europe, Northern Wrens build half a dozen or more nests in each breeding season, although the number is smaller in western North America, as it is also, evidently, in the case of island populations of this species. Similarly, male Northern House Wrens construct multiple nests and, indeed, throughout the breeding season, they seem incapable of resisting the temptation to drop a few twigs into suitable cavities, often to the discomfiture and annoyance of the rightful owners. Among the Campylorhynchus species, the Rufous-naped Wren in El Salvador builds at least eight nests for each one actually used for breeding, at least some of the remainder serving as roosting sites. In Arizona, USA, the male Cactus Wren habitually starts to construct a second nest while his mate is incubating the first clutch; after this brood has fledged and become independent, the female will join in the completing of the second nest, which is immediately used for the rearing of a further brood.
Detailed study, with individually marked birds, is required in order to establish whether a wren species is routinely polygamous. In view of the paucity of such studies with regard to the species not occurring in Canada and the United States, it is not surprising that reproductive strategies are known for only a few of the tropical members of the family. In North America, on the other hand, extensive investigations have revealed that Cistothorus wrens are frequently polygynous. In one Canadian study of Marsh Wrens, undertaken in Manitoba, a total of 120 males consisted of ten bachelors, 53 monogamists, 48 bigamists and nine trigamists, suggesting an unequal sex ratio of about 0·65 males for each female. In contrast, a study in Georgia, in the south-east USA, revealed only 5% bigamous males. Being mated to a bigamist has its disadvantages. For instance, female Marsh Wrens with promiscuous mates tend to receive short shrift from the partner when it comes to providing care for the nestlings, and are frequently left entirely to their own devices. Polygyny occurs also among Sedge Wrens, although less frequently than is the case with the related Marsh Wren. Female Sedge Wrens mated to an already mated male tend to nest later, to lay fewer eggs and to rear fewer young when compared with females in a monogamous relationship, and they rarely attempt a second brood; furthermore, their nestlings tend to weigh less than do those of monogamous parents, no doubt because they receive less food.
Polygamy is known to occur in those species of Troglodytes which have been studied. In western European populations of the Northern Wren, about half of the males are polygynous, although polygyny is apparently rarer among the North American races of this species. Substantial levels of polygyny have, however, been discovered in some populations of the Northern House Wren, especially in situations where surplus nest-sites are available. Secondary females tend to be less successful in rearing young than are primary ones, the latter having the help of the male in the feeding of nestlings. Notwithstanding this, some female Northern House Wrens seem preferentially to choose an already mated male, even when surplus bachelor males are available. Presumably, in those cases, the advantage of passing on one's own genes in combination with the genes of a particularly fecund male outweighs the reduction in the absolute number of offspring produced. In contrast to its northern relative, the Southern House Wren appears to be only rarely polygamous. In one detailed study of this species in Colombia, no instances of polygamy were observed, although, in a Panamanian study, both the active displacement of the male of a mated pair by an intruding male and the opportunistic taking-over of a bereaved female were recorded.
In an interesting study of Northern House Wrens that utilized nestboxes at the University of Wisconsin-Milwaukee Field Station, N. E. Poirier and colleagues found that mate-switching during a single breeding season was fairly common. Although this was initiated by the male, it was not correlated with his paternity of the first brood. It was found that males were more likely to change partner when unmated females were nearby. Such males generally started to advertise for a new mate during the incubation stage or the early part of the nestling stage at their first nest, and they successfully paired with a different female during and just after the nestling period. As mate-switching males sired more or less the same number of young as did males that stayed with the same partner, they appeared not to benefit directly from switching mates. Of the females that were deserted, on the other hand, half did not find a second mate during the breeding season and thus incurred a reproductive cost.
Co-operative breeding by birds is a more common phenomenon than was previously believed, having been documented for a number of different, unrelated avian families. Typically, it involves the assistance of individuals other than the mated pair in defending territories against conspecifics, in rearing young and in defence against predators. The "helpers at the nest", to use the phrase coined by Skutch, are usually blood relatives of the primary breeding pair. By enhancing the survival of that pair's young, the helpers also ensure the propagation of their own genes, albeit in a more diluted manner. Among the troglodytids, co-operative breeding reaches its most developed form in some of the tropical members of the genus Campylorhynchus. In the Stripe-backed Wren of Venezuela and Colombia, breeding groups can contain up to 14 individuals, consisting of a dominant breeding pair and numerous assistants, which are mainly the pair's offspring from previous broods. Several group-members may sing, and all may participate in defensive activities against, for example, Shiny Cowbirds (Molothrus bonariensis); the two dominant individuals, are, however, the only breeders and are the most persistent singers. All members of a group may assist in the rearing of the young, and breeding success is strongly correlated with the number of assistants available. The young wrens usually remain with their natal group for up to a year, after which all females, and the majority of males, disperse to nearby groups, thus reducing the potential for incest. Vacancies in large groups are much sought after and are the subject of much competition; small groups, on the other hand, may have difficulty in filling vacancies. Individuals joining adjacent groups initially have a low rank within that group, but later rise to a senior breeding status.
The Fasciated Wren of western Ecuador and Peru has been less intensively studied than has its close relative the Stripe-backed Wren, but it appears to have a well-developed co-operative breeding strategy, with groups of up to ten individuals for each breeding pair, the young of one year remaining to assist with nesting in the next year. Conventional, unaided pairs do also exist, however. As for other members of the genus Campylorhynchus, the degree of co-operative breeding varies both among species and within a given species. In a Venezuelan study, for example, two different populations of Bicoloured Wrens had quite different strategies. In one population, only 15% of nests had a helper, and then only a single one, whereas more than half of the nests in the other population had assistants, often several of them and of both sexes. Some other Campylorhynchus species show some degree of co-operative breeding, but the strategy seems to be less pronounced in the more northerly species. The Cactus Wren, resident in the southern United States and the northern half of Mexico, is essentially a conventional breeder. On rare occasions juveniles of one brood have been seen to bring food to a subsequent brood, but this seems to be exceptional.
Of particular interest is the reproductive behaviour of the recently described subspecies hernandezi of Apolinar's Wren, which differs from the nominate race in indulging in classic co-operative breeding, with group defence of the territory but only one pair nesting. This appears to be the first report of this reproductive strategy, as opposed to polygamy, for the genus Cistothorus.
Finally, co-operative breeding has been suspected also for the Peruvian Wren (Cinnycerthia peruana). In this case, the variable amount of white feathering on the face of adults may indicate social and breeding status.
With the exception of the cavity-nesting species, and disregarding the atypical Donacobius, all wrens build an enclosed, roofed nest with a side entrance. In the genera Henicorhina, Cistothorus, Troglodytes, Campylorhynchus, Thryorchilus and Ferminia, and in some Thryothorus, the general nest form is an ovoid ball made from grass, rootlets and fibres and with a hole at the side. In the case of the Marsh Wren, the breeding nests are distinguished from surplus or dummy nests by the fact that they have a sill at the entrance, which prevents the eggs from rolling out in windy conditions. The female usually lines the nest with finer material, hair and feathers. In other troglodytid genera, the nests are more elaborate in form, often with an entrance tube pointing downwards. That of the White-bellied Wren, in the monotypic genus Uropsila, is a beautifully woven structure resembling an old-fashioned chemical retort, with an ovoid nesting chamber and downward-pointing entrance tunnel. Some Thryothorus species, such as the Happy and Sinaloa Wrens, build a similar type of two-part nest, often draped over a twig so that the breeding chamber is on one side and the entrance on the other. A similarly shaped nest, suspended over a twig, is made by the Peruvian Wren, but the nests of the other three members of the genus Cinnycerthia have not been described. Likewise, the nest of the Song Wren, the best-known member of Cyphorhinus, is boomerang-shaped, an untidy structure of coarse roots and leaf skeletons placed over a thin branch.
Cavity-nesting wrens do not build a roof over the nest. Males of both the Northern House Wren and the Southern House Wren fill the nesting cavity almost completely with coarse twigs, or even with such items as bits of rusty wire, before the female, after adding more twigs, builds a cup and lines this with hair, wool and feathers. Information on the nests of the four Microcerculus species is scanty, but those described are open structures made from dead leaves.
The crevice-nesting Canyon Wren constructs an open cup of wool, hair and feathers on a base of twigs and moss. Frequently, extraneous matter is included. One especially larcenous Canyon Wren in California regularly pilfered an extraordinary collection of loot from a nearby office, to the total of 600 paper clips, 500 pins, 100 matches and such other miscellaneous sundries as pen-nibs, screws and similar odd items. The American Rock Wren, also a crevice-nester, has the peculiar habit of building a nest foundation of pebbles. The expenditure of energy for this is by no means trivial; one nest of this species, the only member of the genus Salpinctes, had 2·2 kg of pebbles, the equivalent of 120 times the 18-g body weight of the wren itself, with the largest pebble, at 6 g, weighing almost a third of the bird's own weight. The evolutionary advantage of this major expenditure of energy is obscure; curiously, similar behaviour is exhibited by the Black Wheatear (Oenanthe leucura), an inhabitant of rocky slopes in Spain and Morocco. The Thrush-like Wren (Campylorhynchus turdinus) of central South America also frequently includes extraneous objects in its nest, a habit which has given rise to local legend in Bolivia (see Relationship with Man).
In certain cases, the form of the nest is dependent on the selected nest-site. Thus, the Carolina Wren usually builds a domed nest, but in totally enclosed sites, such as nestboxes, it may reduce or omit the roof. Bewick's Wrens, too, will build either an open nest or a domed one according to circumstances.
There is much variation in nest-sites among the Troglodytidae, and sometimes even among members of the same genus. Most wrens nest in vegetation of various kinds, the Thryothorus and some Troglodytes species typically in dense brush, Cistothorus and Ferminia in grasses and rushes, and Campylorhynchus in various bushes and cacti. Other species are cavity-nesters. In North America, for example, the Northern House Wren relies heavily on artificial sites, both nestboxes and such incidental locations as cavities in sheds, but it also uses natural cavities such as woodpecker (Picidae) holes and a variety of other sites; the latter include old nests of paper wasps (Polistes), from which the wren first removes some of the interior structure, and the riverbank burrows of Belted Kingfishers (Megaceryle alcyon), the mud-bottle nests of Cliff Swallows (Petrochelidon pyrrhonota) and the pendent nests of Northern Orioles (Icterus galbula). The petrophilous wrens, those in the genera Salpinctes, Catherpes and Hylorchilus, nest in rock crevices, while the Southern Nightingale-wren, in Microcerculus, utilizes the disused burrows of other species, either in sandy banks or in old termite (Isoptera) nests.
Several members of the family have learnt to place their nests in naturally defended sites. A prime example is that of the Cactus Wren, which makes little effort to conceal its nests, instead building them in the savagely spiny cholla cactus, which acts as an effective deterrent to such predators as raccoons (Procyon lotor) and ring-tails (Bassaricus astutus). Some other troglodytids nest in spiny bromeliads or nettles. Several species, in different genera, routinely site their nests in places where they can obtain vicarious protection, by hostile insects, from such mammalian predators as monkeys. Thus, the Sinaloa and Happy Wrens frequently build in bushes next to the nests of hornets (Vespa). In the American tropics, there exists a curious symbiotic relationship between ants of the genus Pseudomyrmex and bull's-horn acacia trees. The ants nest in the hollow acacia thorns and feed on nectar and small nutritious corpuscles, called "Beltian bodies", provided by the tree; in return for this board and lodging, the ants defend the trees against predatory insects and, further, even remove weeds from the ground around the trunk of emerging plants. Bull's-horn acacias thus represent a very hostile environment, one where the ants are in effect "kept by the acacia as a standing army" and savagely attack the unwary intruder. Several wrens have learnt to capitalize on this, by siting their nests in such trees. The White-bellied Wren in Guatemala almost invariably nests in acacias, and in Costa Rica the Rufous-naped Wren goes even further, actively seeking out trees with the most aggressive ants and defending them against birds of other species. Wrens building in such secure locations appear to make little effort to conceal their nests. In contrast, the Bicoloured Wren, which does not routinely build in ant-inhabited trees, hides its nests much more effectively. Although the presence of wrens' nests would seem to confer no obvious advantage either on the tree or on the ants, the birds and their young are not molested by the insects.
The Troglodytidae exhibit considerable variation not only in nest-sites, but also in egg colour and in clutch size. Egg colour can vary within a genus, and sometimes even within a species. In the genus Thryothorus, some species lay immaculate white eggs or, more rarely, pale blue, bluish-green or deep sky-blue ones, while the eggs of other species are whitish or buff-white with a variable amount of brown or cinnamon speckling or blotching. Similar variation occurs in the genus Campylorhynchus: the eggs of the majority of its members are whitish with speckles or blotches of various shades of brown, but those of the Stripe-backed and Band-backed Wrens are plain white. In the case of the Cactus Wren, even different subspecies lay distinctive eggs, affinis in southern Baja California having paler eggs than those of the races in coastal California, and inland races having egg markings that are more brightly coloured. Similarly, the Sedge Wren lays white eggs in most parts of its huge range, but in Tierra del Fuego the local race hornensis lays speckled eggs, and an Aleutian race of the Northern Wren, meligerus, has white eggs, in contrast to the heavily marked eggs of other subspecies. A further example of intraspecific variation is provided by the Marsh Wren, the eggs of which are usually chocolate-coloured or brown, spotted with darker brown, but some individuals appear to lay clutches of white eggs. Sometimes, single white eggs also appear in a clutch.
There is a general tendency among passerines for clutch sizes to be smaller in tropical regions than they are in temperate ones, while tropical species, perhaps as a means of compensating for this low fecundity, have longer breeding seasons and, often, more broods. This principle seems to hold for most troglodytids, and, indeed, is sometimes apparent even within one species. Thus, the wood-wrens, the Microcerculus wrens and the Cyphorhinus wrens, among other tropical species, lay only two eggs. The clutches of many tropical members of the genus Thryothorus often consist of only two or three eggs, whereas the Happy and Sinaloa Wrens, temperate representatives of the same genus in northern Mexico, usually lay five. For the Cactus Wren, the normal clutch in Arizona is of three to five eggs, rarely up to seven, but the same species in Baja California usually lays just two eggs. Similar variations are found in other species, representing several genera. Frequently, as demonstrated by the Northern Wren and the American Rock Wren, island subspecies have smaller clutches than those of their mainland counterparts.
Wrens are usually multi-brooded, insofar as the local climate allows. In temperate regions, with a constrained breeding season, there is a rush to launch one brood successfully before starting another one. Parental care of the fledged young tends, therefore, to be of short duration in North America and Europe, while in many tropical species the family-members remain together as a feeding party for several months after leaving the nest. This doubtless enhances the survival rates of offspring and thereby compensates for smaller clutches. An exception to this general tropical strategy is provided by the Southern House Wren. Athough the clutch size of this species is small when compared with that of the closely related Northern House Wren, the Southern House breeds rapidly and frequently, one female in Colombia having been observed to raise as many as seven broods in a period of just 17 months.
Incubation of the eggs seems to be the duty exclusively of the female. In come cases, she may be fed on the nest by the male, although in polygynous species, such as the Marsh Wren, she usually receives lamentably poor provision. Incubation periods tend to be longer among the larger species than among the smaller ones. For example, the Cactus Wren sits for up to 23 days and other Campylorhynchus species for 17-20 days, whereas the duration can be as short as 12 days for the American Rock Wren and the Canyon Wren. The incubation period of the small wood-wrens, however, is usually 18-20 days. The nestlings are generally fed by both parents, but in the case of polygynous species the contribution of the male may amount to little or nothing. Among the co-operative breeders, on the other hand, a variable number of helpers assist with brood-feeding.
In the tropics, the young wrens frequently associate with their parents for a prolonged period after fledging, in some cases for almost a year. Parental feeding of the fledglings is probably limited to a much shorter period, but with many species the parents and juveniles remain together as a family unit for months on end. It is easy speculate how such associations may evolve into the co-operative nesting behaviour so typical of the tropical Campylorhynchus wrens; after all, if the time is not right for the individual to reproduce, the next best option for it to propagate its own genes is to enhance the survival of its own siblings, even those of different broods.
Interestingly, in Costa Rica, young Southern House Wrens of one brood have been observed to assist in the feeding of their parents' subsequent broods, even though this behaviour seems to be rare for this Troglodytes species. Among temperate members of this genus, such as the Northern House Wren or the Northern Wren, parental care of fledged young is of short duration, usually lasting for no more than two weeks, after which the juveniles are left on their own. One could speculate that, for migratory species, the necessity of building up fat reserves prior to migration could have a greater survival benefit in terms of "the selfish gene" than does the enhancing of the survival of one's siblings.
The domed nest form of most wrens and the cavity-nesting habit of the remainder appear to provide these birds with some protection from parasitic species. Nevertheless, brood parasitism is a significant factor in reducing the nesting success of certain troglodytids. In some parts of the United States, up to 25% of Carolina Wren nests fail as a result of parasitism by the Brown-headed Cowbird (Molothrus ater). In the Neotropics, the Shiny Cowbird is generally the main culprit in this respect, although it is not a major problem for troglodytids in this region. On the other hand, two Thryothorus species, the Rufous-and-white and Buff-breasted Wrens, are frequent victims of the American Striped Cuckoo (Tapera naevia) in South America, and the Northern Wren in Europe is occasionally parasitized by the Common Cuckoo (Cuculus canorus).
Finally, the nesting habits of the Donacobius exhibit several interesting features, formerly used by taxonomists to argue for this species' inclusion in or, paradoxically, its exclusion from the Troglodytidae. This is a co-operative nester, at least in situations where it occurs at high density. The breeding unit varies from simply the nesting pair alone to groups of up to four individuals. The nest is an open cup, quite unlike that of any other member of the family, and is built in vegetation, often over or at the edge of water. Over most of its range, which extends from east Panama across to the east coast of Brazil and southwards, east of the Andes, to north-east Argentina, the Donacobius lays a clutch usually of two eggs, but in Brazil frequently of three. The eggs, purplish-white and covered by heavy purplish blotches, are incubated by the female alone, for a period of about 16-18 days. A curious habit of the adults is that of cooling the nestlings by wetting them with water brought on the adults' breast feathers. The breeding success of this species is correlated with the amount of help available; pairs without helpers are usually able to rear only one chick to fledging, whereas those with helpers can succeed in raising two or three young. In contrast to most wrens, the Donacobius appears to be single-brooded.
The great majority of wren species appear to be very sedentary. In other words, not only is the species found in a particular location throughout the year, but probably the very same individuals are present there in all months. After leaving the family group, the juveniles will disperse, probably in a random manner, until they encounter a vacant territory. In the case of the co-operatively nesting Stripe-backed Wren, the young of both sexes remain with the breeding group for a year or so, after which all females and the majority of males disperse to breed with nearby groups; the distance in terms of linear displacement may, however, be very short.
Most tropical wrens appear to be sedentary even when faced with major climatic fluctuations, such as the wet and dry seasons of Central America and northern South America. In a few cases, some altitudinal movement appears to occur, although the data are scanty and the precise nature of the movement not clearly understood. For example, the Merida Wren seems to vacate some habitats during the rainy season, and in El Salvador the Rufous-and-white Wren occurs in winter at sea-level in thickets of the palm Bactris subglobosa, a habitat known locally as "huiscoyol", whereas it is not found below 300 m in the summer. At the northern edge of its range, in the Sierra Madre of Mexico, the Happy Wren migrates vertically, but no such movements have been noted farther south in its range.
When one moves into temperate zones, however, it is noticeable that troglodytids become increasingly less sedentary. The North American populations of the Sedge Wren, for instance, are totally migratory, with no overlap at all between the breeding range, which extends from Saskatchewan east to Quebec and south to Kentucky and Pennsylvania, and the non-breeding range, mainly in the coastal states from Virginia south to north-eastern Mexico. These North American populations are also unique in that they are nomadic during the breeding season, abandoning some areas and, later in the same season, appearing in others. In contrast, the Sedge Wrens in Middle America, from the highlands of southern Mexico southwards to Costa Rica, seem to be entirely sedentary. In South America, too, the Sedge Wren populations, which may perhaps represent two separate species (see Systematics), are similarly non-migratory; here, the southernmost subspecies, hornensis, experiences very harsh conditions during the austral winter, although the type specimen of this taxon was taken on a ship "twenty leagues south-east of Cape Horn", suggesting that some movement may occur at times.
A similar distinction in the movements of different populations is exhibited by the Marsh Wren complex of taxa. Those summering in the vast area from northern Alberta eastwards to New England are entirely migratory, spending the winter months along the Atlantic seaboard from New York south to Mexico, whereas the subspecies breeding in coastal habitat in the eastern United States are either partial migrants, in the north, or sedentary. In some coastal marshes in the southern-eastern United States, the Marsh Wren populations present in winter are a mixture of sedentary local breeders and winter visitors from farther north. A totally sedentary race is found in central Mexico. Vagrant Marsh Wrens have been recorded in Bermuda and Cuba, illustrating this species' ability to cross large expanses of open water.
In western North America, the American Rock Wren is partially migratory. It vacates the northern portion of its range, from British Columbia south to Colorado, in winter, whereas the species is present throughout the year in the remainder of the western United States and in Mexico; again, disjunct populations from south Mexico south to Costa Rica are totally sedentary. The American Rock Wren is a notable vagrant, stray individuals turning up, usually in October and November, in various places across eastern North America, from Nova Scotia and New Jersey south to Louisiana and Florida. The Canyon Wren, on the other hand, appears to be much more sedentary. Although it does undertake some altitudinal movements in the northern parts of its range, occurrences of vagrants are fewer, and in localities closer to the breeding area.
A very limited partial migration takes place in the case of Bewick's Wren, the eastern populations of which are more mobile than are the western ones. Carolina Wrens are essentially sedentary, but they do crop up extralimitally, sometimes in locations well removed from the breeding range. Of over 500 recoveries of individuals of this species ringed in North America, only 31 had made any movement at all, and only three demonstrated significant displacement, having covered distances of, respectively, 250 km, 150 km and 75 km. Extralimital movements made by the Carolina Wren may involve wandering by juveniles in their first season.
The two Troglodytes species breeding in North America are strongly migratory. For the Northern House Wren, a large body of data has been amassed from recoveries of ringed birds, and this information allows a detailed analysis of movements; for no other wren species in the Americas are there adequate data for such studies. The Northern House Wren is totally migratory; apart from a small area of southern California, there is no part of its range where it occurs in all months of the year. Individuals ringed on the breeding grounds in coastal eastern North America appear to winter in Florida. Inland breeders, from east of the Appalachians, migrate on a broad front to non-breeding quarters extending mostly from Texas eastwards to Georgia and Florida, those from the eastern part of the breeding range tending to winter farther east than do those from more westerly areas; there are, however, a few recoveries demonstrating strong longitudinal displacements, as, for example, from Ontario south-west to as far as Texas. Northern House Wrens breeding on the Great Plains and the Prairie Provinces of Canada, in contrast, have a strongly south-eastern orientation of migration. There are no significant recoveries of house wrens ringed in western North America. This species has occurred as a vagrant in western Cuba, the Bahamas, Newfoundland, and as far north as Churchill, in the Canadian province of Manitoba.
The Southern House Wren, incidentally, is essentially sedentary over most of its vast range, although it displays a remarkable ability to appear in small isolated patches of suitable habitat when these are created by human activity. The populations in extreme southern South America, however, are truly migratory, leaving southern Tierra del Fuego during the austral winter. The precise whereabouts of their non-breeding quarters are not known, and no information on their movements is available from any ringing studies.
For the other Troglodytes wren of the Nearctic, the Northern Wren, the situation is more complex. The bulk of the breeding range, from northern British Columbia across southern Canada to Newfoundland, is vacated after the breeding season, when the wrens migrate to a wintering range that extends from New England south to Florida, Texas and north-eastern Mexico; the species occurs in all months in southern Ontario and the Appalachian chain, but it is not certain that it is the same individuals that are present there throughout the year. Western North American populations may be partially migratory or sedentary. The numerous endemic subspecies of the Aleutian chain and the Pribilof Islands are known to be totally sedentary, and those breeding in the region from Alaska south through northern California appear to be so. In southern California and Arizona, however, the species does occur in winter but is absent in the summer months.
In the Old World, the Northern Wren again comprises a mixture of sedentary and migratory populations. The species leaves most of Scandinavia and north-western Russia after breeding, as it appears unable to tolerate winter conditions north of the -7°C isotherm. In western Europe, from Iceland south to the Mediterranean Basin, and from the south Baltic region south to Poland and the Ukraine, the Northern Wren occurs throughout the year, as it does also in north Africa, Turkey, Cyprus, the Caucasus and northern Iran. Isolated island populations, such as those on the Faeroes and St Kilda, are also sedentary. Farther east, in the Himalayan region, eastern China, Japan and Taiwan, it is likewise observed in all months. The breeding grounds in northern China and eastern Siberia, including Kamchatka and Sakhalin Island, are vacated in winter, although the species is resident on the Commander Islands, east of Kamchatka.
Whereas there are, unfortunately, no worthwhile ringing data for the Northern Wren in North America, a substantial number of ringing recoveries exists in Europe. Individuals breeding in Norway have been encountered in winter in the region from northern Germany south to Belgium; wrens from Sweden and the Baltic area tend to have a generally, but not exclusively, south-western orientation, with recoveries reported from Hungary westwards, all the way to south-western France and eastern Spain. Northern Wren movements in Britain seem to be of two distinct types. The first is a short-distance movement of up to 50 km, and is essentially random in orientation, while the second is longer, up to 250 km, and with a markedly north-south orientation between summer and winter and the reverse heading in spring. A number of wrens that had been ringed at locations on the south coast of England have been recovered in France, including several in the south of that country. These may well have involved individuals that were already on migration when they were caught for ringing, since it is obvious that the vast bulk of British Northern Wrens, which are subspecifically distinct from mainland European ones, do not leave the British Isles.
Observations of birds striking tall buildings, TV towers, lighthouses and similar structures at night, or being found dead in the morning at such places, indicate that all migratory troglodytid species in North America and Europe migrate nocturnally. This can occasionally lead to considerable mortality (see Status and Conservation).
It is worth mentioning here that the Donacobius, almost certainly not a "true" wren (see Systematics), appears to be a very sedentary species. In one study in Peru, in which birds were individually marked, there was no evidence of any movement between patches of suitable habitat that were separated by a distance of only 2 km.
Relationship with Man
In crass economic terms, wrens have little impact on human beings. Their habitats, whether they be forest, scrubland, marsh or desert, rarely impinge upon agriculture. Apart from a few records of the larger Campylorhynchus wrens attacking soft fruit, the members of this family seldom interact, for good or ill, with farming. There is the possibility that, in certain geographically restricted cases, various wren species may make a modest contribution to local economies in terms of attracting birdwatching visitors. If arrangements for such small-scale tourism were handled properly, species such as the Zapata Wren of Cuba could doubtless bring in minor amounts of hard currency from visiting ornithologists, and a little employment for local people, without further endangering the species itself. Any such economic benefit, however, will always be indeed modest; wrens are, after all, neither lions nor elephants.
In contrast to the minimal economic interaction between the Troglodytidae and humans, the role of wrens in folklore and legend is extraordinarily widespread and pervasive. This is especially true of the Northern Wren in Europe. It is odd that a small, brown, inedible bird should be so important in the folklores of so many diverse cultures. Indeed, in western Europe, among passerines, probably only the European Robin (Erithacus rubecula) figures more prominently. Doubtless, this is due in some measure to the loudness of its song. Certainly, the songs of the members of this family make them difficult to overlook. In Canada, for example, the name for the Northern House Wren in the Ojibway language of Ontario translates literally as "he who makes a lot of noise for his size".
One common feature in many different cultures is the recognition of the wren as the "King of the Birds". In a number of European languages, as diverse as Dutch, Classical Greek, Polish, Romanian and Lithuanian, among others, the vernacular name for the Northern Wren implies royalty. One example is the German Zaunkönig, meaning "king of the fence". In Britain and France, however, the mantle of royalty seems to have been taken over by the even smaller kinglet (Regulus), the roitelet in the French language.
How did the tiny wren become the birds' king? One legend, widespread among several European cultures, tells of a contest among the birds for the throne; the one which could fly the highest would inherit the crown. As might be expected, this was the eagle. As the eagle, exhausted and incapable of further climbing, was about to call down to the unsuccessful claimants below to proclaim his triumph, the wren, which had concealed itself among the eagle's back feathers or, in some versions, under his wing, popped out and flew a few feet higher. The origin of this widespread legend is obscure, but it may have come from one of Aesop's fables, since lost. Aesop himself probably acquired many of his fables from Indian sources, although, curiously, an almost identical legend is found among the Canadian Ojibway people.
An obviously related legend, again widespread in western Europe, involves the bringing of fire, or light, to the world by the wren. This entailed a flight to the heavens from which the wren returned successfully, but with all of his feathers burnt off. In gratitude, all except one of the birds agreed to give the wren one of their own feathers so that he could clothe his nakedness. The single exception was the owl, which maintained that he had none to spare. As punishment for his stinginess, the owl was for ever thereafter condemned to a solitary life; harassed by small birds during the day, and spending the nights alone in the dark forest, making forlorn and mournful hooting noises. A variation of this story identifies the robin as the fire-fetcher, although in Breton legend it was Hell, not Heaven, that provided the fire. In eastern Europe, the fire-bringer was the swallow.
A remarkable tradition, formerly practised in much of western Europe, was the wren hunt. Typically, this took place on St Stephen's day, or Boxing Day, 26th December, although in some locations it happened on Christmas Eve, on Christmas morning, on the Epiphany, or "Twelfth Night", or even on St Valentine's Day. Basically, groups of young boys would catch wrens and attach them to decorated poles, a holly bush (Ilex) or, in Wales, a house adorned with ribbons. The unfortunate bird would then be paraded by a procession of boys and young men in strange and fantastic garb, some dressed as women, sometimes with blackened faces and carrying wooden swords; in Ireland, the parade took place to the accompaniment of doggerel verse and the music of flutes and the bodhrán, a traditional drum. The young participants then went from door to door, soliciting modest funds and treats. In the Isle of Man, the wren would sometimes then be interred with due solemnity and respect, to the sound of dirges sung in the now extinct Manx language. Wren-hunting occurred widely in the British Isles, not only in the Celtic areas but also in such counties as Suffolk and Surrey, as well as in parts of France. The tradition seems to have been strongest, however, in southern Ireland and the Isle of Man. Manx fishermen carried wren feathers with them to sea as protection against shipwreck, while in Scotland fisherfolk scattered wren feathers into the wind in the belief that this would enable them to predict the profitability of the herring (Clupea harengus) harvest.
The origins of the wren hunt are obscure. According to one legend, the wren betrayed St Stephen as he was trying to escape from jail, thereby causing his martyrdom. Several Irish legends exist. In one of these, the wren was cursed by St Moling for eating the holy man's pet fly. In another, a wren presumably of the Protestant faith was blamed for warning King William of Orange's sleeping forces of a stealthy Irish attack, by jumping up and down on a drum; most interestingly, exactly the same story existed seven hundred years earlier, but in this case it was somnolent Danes who were aroused. In all probability, the origins of the tradition date back to early or pre-Christian times. In Celtic mythology, for example, the wren was the king of the oak tree and the symbol of the Old Year, while the robin represented the New Year, hence the symbolic dispatch of the wren at a time close to the winter solstice.
In the New World, wren mythology seems to be less extensive or, in cultures lacking the written word, perhaps simply unrecorded. According to Cherokee legend, the wren closely observes women in labour, rejoicing in the birth of a girl but loudly lamenting the appearance of a boy, who will grow up to hunt the wren. In Suriname, the haunting and arresting song of the Musician Wren is reckoned to be a bad omen for hunters. In contrast, the Southern Nightingale-wren is considered to be a good talisman for deer-hunters in Brazil, although, to be efficacious for this purpose, the bird must be captured on a Thursday or Friday during a period when the moon is waxing. The habit of some species of incorporating human artefacts in their nests (see Breeding) has led to a number of superstitious beliefs. The Thrush-like Wren, for example, frequently includes extraneous objects in its nest and, according to local legend in Bolivia, if one loses a banknote, one should examine all the local wren nests to ascertain whether the thrifty birds have incorporated it into their own nest structures.
Finally, there is the curious fact that, in the Creole languages of the West Indies, the Southern House Wren is endowed not with royalty, but also with divinity. On English-speaking islands it is called "God-bird" and on French ones "Oiseau Bon Dieu", while in Suriname it is known as "Gadofowroe".
Broadly speaking, wrens are looked upon with some affection by the general public. This is presumably due largely to their small size. This may perhaps have been in the minds of the members of the German Union for Nature Conservation when they selected the Northern Wren as the "Bird of the Year" for 2004. That body stated that the Northern Wren, although not threatened, is a symbol of a natural, diverse environment that must be maintained and protected.
Status and Conservation
The fate of different troglodytids at the hands of man is very varied. Some species have greatly benefited from human activity, whereas others have suffered sorely. In North America, the clearing of continuous climax forest during the period of European settlement created the forest-edge habitat suitable for the Northern House Wren and Bewick's Wren, while artificial structures provided secure nesting sites, allowing for major expansions in the range of both species. In the United States, Northern House Wrens colonized western West Virginia by the late 1800s, North Carolina by the 1920s and South Carolina by the 1940s. Bewick's Wren had expanded into West Virginia by 1834, Pennsylvania by 1843, Georgia by 1854, Ohio by 1879 and New Jersey by 1890, and had reached southern Ontario, in Canada, by 1898. In contrast to the house wren, however, Bewick's Wren has suffered major declines in the eastern part of its range and now occurs only in isolated pockets east of the Mississippi; it has been proposed that the race concerned, altus, be listed as Endangered. The reasons for this major decline are not entirely clear, but one plausible explanation is the well-documented nest destruction by Northern House Wrens (see General Habits). The latter may, in fact, represent a conservation problem under some circumstances. In Canada, the cavity-nesting Prothonotary Warbler is confined to southern Ontario, the entire population totalling fewer than two dozen pairs. Its nesting success is adversely affected on a substantial scale by Northern House Wrens, being significantly higher in areas without these nest-destroyers. The wrens cause nesting failure by puncturing and sometimes throwing out the warbler's eggs, attacking nestlings and perhaps adults, and dropping twigs into nest-cavities already occupied by the warblers. The decline of Bewick's Wren in eastern North America seems to have coincided with the expansion of the range of the Northern House Wren. Although the attributing of the disappearance of Bewick's Wren to the Northern House Wren's activities may be a facile explanation of a more complex phenomenon, the evidence is suggestive.
The two Cistothorus wrens of North America have rather specialized habitat requirements, the Marsh Wren occurring in reeds, rushes and grasses over water and the Sedge Wren in damp sedgy meadows. Since Europeans settled in North America, Marsh Wren habitat has been extensively destroyed as wetlands have been drained for agriculture and other purposes. The species' total population has doubtless declined substantially, but it remains common or abundant in suitable habitat, which in some areas is now expanding as a result of the building of artificial water impoundments. It is probable that the Sedge Wren was helped by the creation of habitat through human clearances; this wren may also breed in weedy rice fields, although these are discouraged in modern farming practices. Nevertheless, Sedge Wrens are regarded as threatened in several US states. In Canada, breeding-bird surveys over the decade 1991-2000 indicate a slow population increase.
No species of wren has become extinct in historical times, but several give cause for concern, and a number of subspecies have indeed disappeared. Of these, the race exsul of the American Rock Wren, restricted to the island of Isla San Benedicto in the Revillagigedo Group, off western Mexico, became extinct in the most spectacular fashion in 1952, when its island home catastrophically erupted. A few wrens survived the initial eruption, which also killed tens of thousands of nesting seabirds, and lingered for a few months, but all ultimately perished. All other extinctions have been caused by man, usually by the sadly familiar practices of habitat destruction and the introduction of alien predators. Two island subspecies of Bewick's Wren have been lost: brevicauda was one of several Guadelupe endemic taxa that vanished from that island, off Baja California, prior to 1892, and leucophrys, of San Clemente Island, California, became extinct by, probably, 1986. In the West Indies, the Martinique race of the Southern House Wren became extinct about 1900. Two other races of that species, guadeloupensis on Guadeloupe and mesoleucus on St Lucia, are hovering on the brink, if they have not already gone over it; the first was believed to be extinct, but a tiny population was rediscovered in 1969 and 1973, and the second still persisted, in very small numbers, in 1984. No recent information is available for either subspecies. On some other islands, such as Dominica, the local races of the Southern House Wren remain common. Given the ability of this species, over most of its range, to adapt to or, indeed, to benefit from human-wrought changes, the loss of some of the races is perhaps unexpected. It can doubtless be ascribed to the typical vulnerability of naive island populations to introduced predators, in these instances predominantly mongooses (Herpestidae).
Populations of some island races of the Northern Wren fluctuate, at times plummeting to alarmingly low levels. In Scotland, for example, the subspecies fridarensis, confined to the 7·6 km² of Fair Isle, between the Shetlands and the Orkney Islands, has numbered as few as ten singing males, but it rapidly bounced back to three or five times this level. When the taxon hirtensis was first described on St Kilda, the westernmost of the Outer Hebrides, in 1884, it was initially considered to be a new species; this unleashed a frenzy of collecting, with single skins fetching a guinea or more, the equivalent of £1.05 in modern British currency, which was at that time a huge sum of money for an impoverished local Gael. Notwithstanding this, hirtensis, now treated as a subspecies of the Northern Wren, was probably never in danger, mainly because of the physical inaccessibility of much of the population, and nowadays it typically numbers more than 200 pairs. At the eastern edge of the Northern Wren's Palearctic range, the subspecies mosukei on the Izu Islands, south of the Japanese mainland, is considered endangered, while the subspecies orii recently became extinct on the Daito Islands, about 800 km south of Kyushu.
Several troglodytid species are confined to islands of various sizes, but, in contrast to the sad condition of the West Indian races of the Southern House Wren, all of them should survive. The Cozumel Wren (Troglodytes beani) is restricted to the Mexican island of that name, where it is generally both abundant and tolerant of moderate habitat modification. Off the other, western coast of Mexico, in the Revillagigedo Archipelago, are the much smaller islands of Clarión and Socorro, each with an endemic wren. Because of the small size of their respective ranges, the Clarion Wren (Troglodytes tanneri) is classified by conservationists as Vulnerable and the Socorro Wren is considered to be Near-threatened. Both islands have suffered substantial habitat changes following the introduction of such animals as goats and cats, but the wrens appear able to tolerate this environmental alteration. This situation contrasts with that of the Socorro Mockingbird (Mimodes graysoni), which is in a very sorry state and is listed as Critically Endangered. Fortunately, the archipelago is now a biosphere reserve, and efforts are being made to remove alien species.
In the Falkland Islands, the endemic Cobb's Wren was probably widespread throughout prior to human settlement, but it has since retreated to offshore islands in the face of habitat destruction and the introduction of cats and rats (Rattus). Although classified as Vulnerable owing to its restricted range, it has an estimated population of 4000-8000 pairs, spaced over more than thirty islands of between 5 ha and 3000 ha, and does not seem to be in any real danger. Accidental introduction of either cats or rats to any of these islands, however, would almost certainly result in the disappearance of the local wren population. Efforts are currently being made to exterminate alien predators on several of the more suitable islands.
Most of the conservation worries concerning troglodytids refer to Central and South American members of the family. No fewer than four species of concern occur in Colombia, a country in which an almost permanent state of severe civil unrest is not conducive to conservation efforts in remote regions. The species at the most serious and most immediate risk is Niceforo's Wren (Thryothorus nicefori), which was described in 1945 from a small area near Bucaramanga and was then not seen again until 1989, when one pair was observed. It appears to be dependent on acacia scrub, a habitat which is highly disturbed by burning, overgrazing and clearance for agriculture. Niceforo's Wren is presently classified as Critically Endangered.
Apolinar's Wren, "officially" listed as Endangered, is restricted to tiny patches of habitat in Colombia, where recent study has revealed the existence of two separate populations. These are currently treated as representing subspecies, but each is perhaps deserving of full species status (see Systematics). The nominate race is found only in reedbeds around a number of high-altitude lakes. Unfortunately, many of these are in heavily populated areas in the departments of Cundinamarca and Boyacá, and much suitable habitat has been lost in recent years. Populations of 50 or so pairs exist at Laguna de Tota, in Boyacá, and at Laguna de Fúquene, in Cundinamarca; the species apparently still persists in some locations around Bogotá, and the type specimen was, in fact, collected only about 6·5 km from the capital city. Some of the breeding habitat of this taxon lies within national parks, but, given the current state of civil unrest in Colombia, park status may be no guarantee of protection. Recent estimates of the total population of the nominate race are in the region of 250 adults, justifying for this taxon a classification of Critically Endangered. A further cause for concern with this race involves a growing incidence of brood parasitism by the Shiny Cowbird, the local population of which appears to be increasing. The other subspecies of Apolinar's Wren, hernandezi, has so far been found in just two small locations. Its population density at these seems to be quite high, and the sites themselves do not appear to be under immediate threat. It is feasible that this race may occur also at other places with suitable habitat, but, because of concerns over security, it has not yet been possible to investigate these. In view of the tiny currently known range of hernandezi, it is suggested that it be afforded the conservation category of Vulnerable.
Although the Santa Marta Wren is not "officially" listed as globally threatened, it has a very limited total range, being confined to higher levels in the massif of Santa Marta, in north Colombia; notwithstanding the supposedly protected status of much of its range, which is encompassed within a national park, habitat destruction, including overgrazing, burning and forest-cutting, continues apace, suggesting that the species should be designated as, at best, Vulnerable. Similarly, the recently discovered Munchique Wood-wren (see Systematics) has a very small range of less than 100 km², greatly constrained climatically, not far from the city of Popayán, in the Western Andes. Despite the fact that its entire range lies within two protected areas, lack of adequate funds for effective wardening to prevent habitat destruction for agriculture gives reason for grave concern over its future, perhaps sufficient to recommend its being placed in the conservation category of Critically Endangered. By way of contrast, the Merida Wren of the Venezuelan Andes, despite its very restricted range, does not appear to be at any risk.
The two members of the Mexican genus Hylorchilus might almost be regarded as island species in that they live only in forest growing on isolated outcrops of karst limestone. Whether they ever occurred in the intervening non-karst areas is something of a moot point, since these have been largely deforested for agriculture. Both species are the subject of some concern. Sumichrast's Wren seems to be in the better shape. Although its total range is encompassed in an area of only 6000 km², in only a small proportion of which it is actually present, a total of twelve separate populations has been located. In some of these it is found at the very high density of one pair per hectare, and it is able to exploit very small patches of habitat, some as tiny as 0·5 hectares. In addition, it tolerates a degree of habitat disturbance. Sumichrast's Wren is currently considered Near-threatened, the greatest threats facing it being the cutting of forest for firewood and quarrying. Nava's Wren is classified as Vulnerable, with suggestions that it should be upgraded to the conservation category of Endangered. It appears to be less tolerant of habitat disturbance than is its congener. Some parts of its range are protected, but the viability of both species can be guaranteed only by further land protection, either by direct acquisition for reserves or through long-term agreements with landowners.
Farther south, in El Salvador, the local subspecies nannoides of the Rufous-browed Wren is now restricted to a protected area of 600 ha on Volcán Santa Ana. Here, its population is estimated to number no more than about 4000 individuals.
The unique Zapata Wren, confined entirely to small areas of the Zapata Swamp, in south-western Cuba, is a source of very serious concern. It is currently classified as Endangered, and at one point it was feared that it was already extinct. Recent observations, however, suggest a population in the region of 80-100 pairs. The main threats to this species appear to be destruction of its habitat by fires and predation by introduced mongooses. Currently, management plans for the Zapata Swamp are in place, with the aim of preserving this and several other endemic species.
Twenty-one troglodytids, including five of the six globally threatened ones, are restricted-range species. Of the non-threatened species in this category, it is believed that their small geographical ranges would render them vulnerable in the event of major habitat destruction. In southern Mexico, for instance, the Giant Wren is restricted to a narrow coastal strip of the state of Chiapas. It seems to be abundant in parts of this small range, and to be able to co-exist well with present land uses, but any change in its environment could cause problems for this distinctive wren. On the Caribbean coast, the Yucatan Wren (Campylorhynchus yucatanicus) occupies an even smaller range, some of which is protected by national parks. This, however, is no justification for complacency, as the Yucatán Peninsula is currently undergoing extensive development for resorts. As a further example, the Bar-winged Wood-wren, already listed as Near-threatened, occurs in very limited areas on high mountains in southern Ecuador and northern Peru, where it is confined to stunted forest on impoverished soils. This choice of habitat, unattractive for human exploitation, is probably the species' best guarantee for survival. Also highly restricted in Peru is the Inca Wren, which does, however, seem to be common in suitable areas.
Although many members of the family have large geographical ranges, these are becoming increasingly fragmented. This is especially true of species which rely on lowland tropical forest and have limited tolerance of change, such as the Northern Nightingale-wren, large segments of the former habitat of which have been deforested. While such species may be abundant in areas where suitable habitat still remains, their populations as a whole have exhibited substantial declines in recent years. The future health and viability of these forest-reliant species will obviously depend on the capacity of the human population to reverse the present rate of habitat destruction in Central and South America.
Apart from habitat loss and degradation, there appear to be few threats to troglodytids. As mentioned previously (see Movements), non-resident populations of wrens migrate during the night-time. These nocturnal movements can at times involve certain hazards, chief among which is probably the risk of flying into stationary structures such as lighthouses, TV towers and various tall buildings. Observations suggest that, under certain weather conditions, Northern House Wrens, Marsh Wrens and Sedge Wrens in North America and Northern Wrens in Europe can suffer substantial numbers of casualties as a consequence of hitting tall obstructions in the dark. The level of mortality due to such accidents, however, is extremely minimal in terms of the total population of the species concerned.
Natural deaths resulting from starvation during prolonged periods of harsh weather can be more serious, sometimes depressing populations for several years. Nevertheless, wrens are generally capable of recovering to their former strength within a relatively short length of time.
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