HBW 6 - Species accounts: Common Barn-owl (Tyto alba)

10. Common Barn-owl
Tyto alba

French: Effraie des clochers German: Schleiereule Spanish: Lechuza Común

Other common names: Barn Owl, Lesser Masked-owl(!), Monkey-faced Owl, Cave/Death/Delicate/Demon/Ghost/Golden/Hissing/Night/Screech/Silver/White Owl; Australian Barn-owl (delicatula); Galapagos Barn-owl (punctatissima).

Taxonomy. Strix alba Scopoli, 1769, Friuli, Italy.
Up to 46 races recognized in recent works, but status and distribution of several uncertain, and review of whole group long overdue. West Indian taxa glaucops, insularis and nigrescens formerly included in present species, now shown to be separate species, T. glaucops. Considerable variation in size and colour may be more individual than geographical in many continental and some island regions, with possibly expanding zones of intergradation, particularly in Europe. Validity of many proposed races considered too doubtful to be upheld: in Europe, pusilla, kleinschmidti, kirchhoffi and hostilis merged with nominate, and rhenana with guttata; Madagascan hypermetra with African affinis; island populations everetti, kuehni, bellonae, lifuensis and lulu with widespread Australasian delicatula; Bahamian lucayana with pratincola; and, in Neotropics, Colombian subandeana with guatemalae, stictica with contempta and hauchecornei with tuidara. Also, current separation of niveicauda from furcata (based on colour characters) and of guatemalae from pratincola (ranges in Central America not yet satisfactorily determined) perhaps doubtful, and continued separation of bondi may not be tenable; size and colour variation in Neotropics in need of reassessment, and separation of hellmayri from tuidara based on size characters questionable. Furthermore, poensis possibly not separable from affinis, in which case poensis (as older name) has priority over latter. N African nominate race birds sometimes placed in erlangeri; birds from S Myanmar to Indochina sometimes placed in javanica, but seem better included in stertens. Melanesian populations, currently covered by 3 races (delicatula, crassirostris, interposita), may merit further splits. Several well-marked insular races (e.g. detorta, thomensis, deroepstorffi, punctatissima and possibly others) may be separate species; recent proposals to elevate delicatula to species level, however, do not consider which races it would include (see page 64). Twenty-eight subspecies currently recognized.
Subspecies and Distribution.
T. a. alba (Scopoli, 1769) B W & S Europe (including Balearic Is and Sicily) to N Turkey; also W Canary Is (Tenerife, Gran Canaria, El Hierro), and N Africa from Morocco to Egypt (except Sinai), S to N Mauritania, S Algeria, Niger (Aïr Massif) and N Sudan.
T. a. guttata (C. L. Brehm, 1831) - C Europe E to Latvia, Lithuania and Ukraine, grading into nominate race from N & E France to Germany (Rhine Valley) and from C Switzerland to former Yugoslavia.
T. a. ernesti (Kleinschmidt, 1901) - Sardinia and Corsica.
T. a. erlangeri W. L. Sclater, 1921 - Crete and smaller S Greek islands, Cyprus and patchily from Syria E to W Iran and S to NE Egypt (Sinai) and S Arabian Peninsula.
T. a. schmitzi (Hartert, 1900) - Madeira and Porto Santo.
T. a. gracilirostris (Hartert, 1905) - E Canary Is (Fuerteventura, Lanzarote, Lobos, Montaña Clara, Alegranza).
T. a. detorta Hartert, 1913 - Cape Verde Is.
T. a. affinis (Blyth, 1862) - Africa S from S edge of Sahara, including Zanzibar and Pemba, and Madagascar and Comoro Is, intergrading with nominate race in region of S Egypt and N Sudan.
T. a. poensis (Fraser, 1842) - Bioko I.
T. a. thomensis (Hartlaub, 1852) - São Tomé I, and recently recorded on Príncipe I.
T. a. stertens Hartert, 1929 B Indian Subcontinent E to Vietnam, and S to N Sri Lanka and S Thailand.
T. a. deroepstorffi (Hume, 1875) - S Andaman Is.
T. a. javanica (J. F. Gmelin, 1788) B Malay Peninsula S to Greater Sundas (including Krakatau, Pulau Seribu and Kangean Is, and possibly S Borneo) and E to Alor, as well as Tanahjampea, Kalao and Kalaotoa.
T. a. sumbaensis (Hartert, 1897) - Sumba I.
T. a. meeki (Rothschild & Hartert, 1907) - E New Guinea and nearby islands of Manam, Karkar and Umboi.
T. a. delicatula (Gould, 1837) - Sawu, Roti(?), Timor, Jaco, Wetar, Kisar and Tanimbar Is; Australia and offshore islands; possibly New Britain and New Ireland; also Nissan, Buka, Solomon Is (including Bougainville), S Vanuatu (Erromanga, Tanna, Aneityum), New Caledonia, Loyalty Is, Fiji (N to Rotuma), Wallis and Futuna Is, Niue I, Western Samoa and Samoa.
T. a. crassirostris Mayr, 1935 - Tanga Is (Bismarck Arch.).
T. a. interposita Mayr, 1935 - Santa Cruz Is, Banks Is, N Vanuatu (S to Efate).
T. a. pratincola (Bonaparte, 1838) - S Canada S at least to Mexico; also Bermuda, Bahamas and Hispaniola.
T. a. guatemalae (Ridgway, 1873) B Guatemala and perhaps S Mexico to Panama (including Pearl Is), possibly to N Colombia.
T. a. bondi Parkes & Phillips, 1978 - Bay Is (Roatán, Guanaja), off N Honduras.
T. a. furcata (Temminck, 1827) - Cuba, Cayman Is and Jamaica.
T. a. niveicauda Parkes & Phillips, 1978 - I of Pines.
T. a. bargei (Hartert, 1892) - Curaçao, and possibly also Bonaire.
T. a. punctatissima (G. R. Gray, 1838) - Galapagos Is.
T. a. contempta (Hartert, 1898) - W Venezuela, Colombia (except N?), Ecuador and Peru.
T. a. hellmayri Griscom & Greenway, 1937 - E Venezuela (including Margarita I) through the Guianas to N Brazil (S to Amazon); also Trinidad and Tobago.
T. a. tuidara (J. E. Gray, 1829) B Brazil (S of Amazon) S to Tierra del Fuego and Falkland Is.
Introduced (affinis) to Seychelles; also (delicatula, pratincola) to Lord Howe I, where now presumed extirpated (few records since 1970’s considered vagrants); also (pratincola) to Hawaii.

 

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Descriptive notes. 29-38 cm; 187-455 g (Europe, N Africa), 266-470 g (S Africa), 400-700 g (North America), 387-558 g (Surinam), 264 g (Galapagos Is); male av. 555 g, female av. 612 g (Malaysia); male 227-418 g, female 220-475 g (Australia). Medium-sized, long-legged owl with distinctive heart-shaped face. Nominate race golden-buff above, with variable light greyish Aveil@, and finely streaked, mottled and dotted dark; white facial disc and underparts, sometimes with pale buff on sides of chest and/or fine spotting on breast and flanks; legs densely feathered; eyes dark; buoyant, slow, wavering flight, with legs dangling, appears ghost-like. No similar owl over much of range (but in Afrotropics race affinis overlaps with T. capensis and from India to Australia stertens and delicatula with T. longimembris, both those species longer-legged and usually darker, also in Australia with T. novaehollandiae, which usually larger and darker but some more like present species). Female like male, tends to be slightly darker. Juvenile similar to adult, or more heavily spotted. Racial variation considerable, from light grey to buff above and from white to buff below, with varying amount of black spotting and speckling, but widespread intergradation (common in Europe and tropical regions), and island races tending to be smaller and with either whiter or buffier plumage (but some isolated, dark tropical island races): guttata grey, sometimes buff-washed, above, buff (male) to rufous-buff (female) and usually spotted below; ernesti pale and white-breasted; erlangeri also white-breasted, but upperparts more golden; schmitzi, gracilirostris and detorta buff-breasted, darkest in detorta; affinis greyer above than nominate, with more and coarser spotting below; poensis similar, but darker above, more golden-buff and streaked and speckled black and white, primaries barred and tail buffier; thomensis smaller, much darker, dark grey above with conspicuous small black and white spots, golden-brown below; javanica more golden-buff above, larger spots below; stertens like javanica, but paler and greyer above, smaller spots on pale underparts; deroepstorffi smaller, darker, with dark brown upperparts, deep brownish-rufous underparts; sumbaensis slightly larger than javanica, larger bill, almost white tail with narrow black bars; meeki like sumbaensis but tail bars paler, bill smaller, underparts silvery-white with arrow-shaped dusky spots; delicatula like javanica, but brownish-grey to light grey above, mottled pale brown with slight tawny tinge, white below, 4 brown bars on tail; crassirostris darker, bill and feet more robust; interposita washed orange-ochre; pratincola upperparts vary from pale orange-buff to lighter or darker grey mixed with orange-buff, underparts white to pale orange, spotted or vermiculated brown (similar to European birds but larger, with stronger legs and feet); guatemalae like darker pratincola, but more uniform above, more coarsely speckled below (extent of differentiation from pratincola indeterminate); bondi apparently smaller and paler than pratincola; furcata pale orange-buff above with greyish-brown spots, white below with few flank spots; niveicauda averaging whiter and paler than furcata; bargei like nominate, but smaller, with shorter wings; punctatissima small, dull brown above with few scattered dusky white spots, golden-buff to white below with brownish vermiculations or fine dense spots; contempta black above with fine pale grey mottling and white spots at feather tips, pale rusty-brown below with irregular black cross-markings, but characters variable; hellmayri larger than tuidara, both dark to pale yellowish above and near-white to golden-buff below, with or without dusky spotting. Voice. Diverse range of calls associated with breeding, including screeches, wheezes, purrs, snores, twitters, hisses and yelps; less vocal at other times; also bill-snapping, tongue-clicking and wing-clapping. Male’s familiar screech, considered advertising or warning call, usually given in flight, a hoarse, screaming, eerie Ashrrreeeeee@ with gargling or rattling quality and tremulous effect (from wing action), of c. 2-3 seconds’ duration, repeated after 1-20 seconds, often many in series, but usually singly when patrolling territory; female’s variant lower-pitched, more broken, especially when both call during aerial chases. Variations of other loud screams and screeches attributed to mobbing response or distress behaviour.
Habitat. Occurs in great variety of habitats according to availability of prey, seasonality in temperate regions, and competition from other predators; in higher latitudes limited by severity of winters, rarely occurring N of areas with mean Jan temperature around or few degrees below 0EC. Prefers open lowlands with some trees, including farmland with hedges, ditches, ponds and banks, roadside verges and related rougher terrain, and young conifer plantations; also around towns, suburbs, villages or more isolated buildings suitable for daytime roosts and nest-sites; sometimes near refuse dumps. In lower latitudes, also semi-arid and some arid regions with xerophytic vegetation, dwarf shrub and herb communities, deciduous or mixed eucalypt woodland, Acacia savanna, thornbush, heathland, open marshes, mudflats, oil palm plantations, irrigation areas, rice paddies and cane fields, and cliffs and rocky coasts in some regions, notably on continental offshore islands; some darker island races also in forest; on small tropical islands in all available habitats. Usually lowlands, but at higher altitudes in many areas, up to c. 4000 m.
Food and Feeding. Diet better studied than that of any other raptor, most extensively in Europe and North America but also in Israel, parts of Africa (particularly S Africa) and South America (Galapagos Is, Chile, Argentina, Surinam), Australia, and a few other localities. In major sample studies, 74-100% (most 90-100%) of diet small mammals, usually dominated by only few species, especially rats and mice; in Europe and North America also voles (Cricetidae), gophers (Geomyidae) and shrews (Soricidae); in Africa also shrews, other Insectivora and gerbils (Gerbillinae); in South America also small opossums (Didelphidae); in Australia also bandicoots (Peramelidae), dasyurid marsupials and gliders (Burramyidae/Petauridae). Other mammals include young rabbits and hares (Leporidae), moles (Talpidae), bats, and skunks, stoats and weasels (Mustelidae). Also birds, particularly communally roosting passerines, e.g. thrushes (Turdidae), starlings (Sturnidae), sparrows (Passeridae), weavers (Ploceidae) and finches (Fringillidae), but vast range of other birds in small numbers or of local importance, from seabirds (Procellariidae, Hydrobatidae, Sternidae, Alcidae), small herons (Ardeidae), kestrels (Falco), young megapodes (Megapodidae), quails (Phasianidae), domestic fowl, buttonquails (Turnicidae), rails (Rallidae), plovers (Charadriidae), small waders (Scolopacidae), to pigeons (Columbidae), lorikeets (Psittacidae), cuckoos (Cuculidae), swifts (Apodidae), kingfishers (Alcedinidae) and small woodpeckers (e.g. Colaptes), and wide range of passerines from at least 12 families. Other prey lacertid and agamid lizards, skinks (Scincidae), geckos (Gekkonidae), chameleons (Chamaeleonidae), slow-worms (Anguis fragilis) and grass snakes (Natrix natrix); frogs (Anura) and toads (Bufonidae); small fish, e.g. trout (Salmo), roach (Rutilus), carp (Cyprinidae), rudd (Scardinius erythrophthalmus), perch (Perca) and grunion (Leuresthes tenuis); larger insects, e.g. grasshoppers (Saltatoria), beetles (Coleoptera), mantises (Mantidae), moths (Lepidoptera), wasps (Hymenoptera) and termites (Isoptera); spiders (Arachnida) and scorpions (Scorpionida); littoral isopods (Isopoda); earthworms (Annelida); and rarely carrion. Much non-mammalian prey caught incidentally or opportunistically, but can constitute significant part of diet in some tropical and semi-arid regions, e.g. lizards and invertebrates in parts of Africa and on some islands. Discarded eggshells and faeces of smaller nestlings may also be eaten; occasional chick cannibalism at nest, victim usually dead from unknown causes, but young recorded killed and eaten by siblings. In some temperate regions, when main prey species scarce or absent, many starve rather than switch to alternatives; this also occurs in boom/bust cycle of rat and mouse plagues, when excess production of young leads to many often starving. Hunts close to ground in searching flight, undulating and hovering, diving on to prey with talons extended, usually from height of c. 3 m, also commonly from perch; either method used by same bird, or one more than other, depending on habitat type, e.g. perch-hunting predominant in denser vegetation. Normally strictly nocturnal, with high auditory acuity in locating prey in complete darkness. Daylight hunting regular in Scotland and England and well known on various Pacific islands; also observed elsewhere, e.g. W USA (Utah) and Surinam, though extent unknown; may be widespread if owls not mobbed when they appear. Normally hunts singly; sometimes many together during prey plagues, e.g. in Australia, where often joined by T. longimembris, T. novaehollandiae and other raptors.
Breeding. In tropics can start in almost any month, generally late in dry season, in more arid areas during wetter periods; in N temperate regions mostly Mar-Jun, when double-brooded mostly Mar-May and Jun-Aug, initial activity varying among habitats; in some temperate regions season correlated with cycles of prey abundance, but similar from year to year where highly dependent on more homogeneous agricultural landscapes; in South Africa after rains, Feb-May in summer rainfall regions and Aug-Dec in winter rainfall region of SW Cape, but can breed in any month depending on local conditions; in Australia opportunistic according to food supply, can be at any time of year, most peaking Mar-Jun and Aug-Nov; May-Aug in New Caledonia. 1 or 2 broods, rarely 3 (e.g. in Malaysia), depending on food supply; exceptionally, 4 (Zimbabwe) within 11 months during mouse plague, with first egg of next clutch laid while chicks still in nest. Monogamous, but polygamy sometimes recorded, rarely polyandry and incest; territorial, but loosely colonial nesting recorded in USA; sometimes near other owls, e.g. in Scotland 34-40 m from Tawny Owl (Strix aluco) and in USA in association with Great Horned Owl (Bubo virginianus); in captivity, hybridization recorded between male of present species and a female Striped Owl (Asio clamator), 2 of 4 eggs fertile and developing until 15th day. Site a natural cavity in tree trunk, stump or large hollow branch, 2-20 m above ground, or in cliff or bank, including sea-cliffs and small, exposed islands, in cave, walls of lava tubes, even in ground; also in hollow of palm tree; wide diversity of artificial structures with adequate room and access also used (e.g. old church, tower, castle, ruin, barn, abandoned building, loft, dovecote, windmill, disused water tank, around bridges and other large structures, lighthouse, chimney, haystack, duckblind, unused boat, up to 10 m down in abandoned well); also purpose-built nestboxes and those used in rat-control programmes (e.g. in Malaysia); burrows c. 1 m deep in cliffs and banks may be wholly or partially excavated; also uses large abandoned nest, e.g. of Osprey (Pandion haliaetus), in Africa of Hamerkop (Scopus umbretta) and communal nests of Sociable Weaver (Philetairus socius); sometimes takes over occupied nest, e.g. in Scotland wool-lined stick nest of Eurasian Jackdaw (Corvus monedula) in chimney, yet also recorded sharing active site with latter; some sites used over multiple seasons; either no nest or slight scrape or depression, with pellets and other debris as insulating layer. Clutch 2-14 eggs, usually 4-7, laid 2-3 or more days apart; if first clutch lost, replacement usually smaller; incubation 29-34 days; male’s only involvement is in feeding female and young; natal down white, second (mesoptile) down white, creamy-white or buffish-cream; nestling period 7-10 weeks; young dependent on parents for further 3-5 weeks, then disperse within 2-8 weeks. Most hatched chicks survive. Can breed in less than 1 year.
Movements. Most populations sedentary, with post-breeding dispersal of juveniles, as well as some movement of adults where prey numbers fluctuate. Data from Europe and North America indicate considerable regional variation, particularly after high prey abundance. In Scotland most disperse within 20 km; in C Europe 50-100+ km to W & S, with a few ringing recoveries 1200-1600 km distant; Dutch-ringed young moved c. 1500 km to Spain and Ukraine; nominate race vagrant to Azores, guttata to S Scandinavia and S Finland, each also vagrant within other’s range. In America, pratincola from N USA move generally farther, 80-320 km, with recoveries at up to 1760 km; in S USA most disperse within 80 km, but recoveries at 248 km and 984 km indicate that wide dispersal not entirely due to severe winter weather; wanders or strays seasonally to Newfoundland, S Alaska, Tres Marías and Revillagigedo Is off W Mexico, Cuba, Hispaniola, Puerto Rico and Central America. Reported evidence of migration in NE USA suggested as flocking behaviour associated with post-breeding dispersal. Middle Eastern race erlangeri vagrant to NE Iran/Transcaspia region. Movements of Afrotropical affinis include 1000 km from Senegambia to Sierra Leone and 400 km within S Africa. Indian/Indochinese race stertens vagrant to SE China (Yunnan); records from southernmost Thailand since 1970’s may be N-moving javanica rather than stertens (javanica formerly only vagrant W of Java, with spread as breeding bird only since 1960’s). Movements of 65-840 km recorded for delicatula in SE Australia, where some populations highly nomadic, with irruptions associated with rodent plagues, as well as seasonal movements to N areas for dry season (winter) and away from coasts in summer, doubtless accounting for long-distance vagrancy to Norfolk and Lord Howe Is and New Zealand. However, several New Zealand records are of birds transported in undercarriage bays of aircraft (1, dead on arrival, apparently from USA), and some may originate from Fiji; the 7 or 8 records include 1 on Little Barrier I, Jun-Oct 1992. Race delicatula also irregular visitor to Tasmania and Bass Strait islands and vagrant to Houtman Abrolhos off W Australia. In S South America tuidara has occasionally reached Falkland Is, where breeding first reported 1989. Over-water dispersal is obvious source of many vagrants; several records of owls landing on ships up to 360 km from land suggest some vagrancy ship-assisted, e.g. a New Jersey bird recovered in Bermuda (c. 1250 km), where first bred 1931; also sightings on offshore oil rigs. In some tropical areas inter-island movement common, e.g. in smaller island groups of Fiji. Unidentified dark Tyto owls recently observed on several Fiji islands, if not vagrant T. longimembris, could be dark-morph birds of present species, hitherto unknown from Fiji.
Status and Conservation. Not globally threatened. CITES II. Status of many populations uncertain, particularly those on islands, but others locally common; species expanding in some areas, especially temperate regions, as a result of increases in availability of suitable habitats and artificial nest-sites. Formerly occurred N to S Scandinavia, and small numbers have been reintroduced to S Finland; recently extinct in Malta; has also disappeared from Aldabra; recorded in error from Austral Is and Society Is; recent breeding records in Falkland Is. Current European population 110,000-230,000 breeding pairs, with c. 90% of nominate race and two-thirds of total in just 2 countries (France and Spain); densities of up to c. 25 pairs/50 km5 but generally much lower, especially in E Europe. Far greater densities attained in some nestbox programmes, e.g. on oil-palm estates in Malaysia. Nestboxes used as part of local conservation efforts in Europe and North America, where N populations have been declining for c. 50 years; causes of decline include loss and fragmentation of grassland foraging habitat, intensification of agricultural practices since 1940’s, urbanization, and road development, with attendant upsurge in road mortalities, all coupled with increasingly harsh winters during that period. Increased mechanization of farms has meant loss of important foraging sites, such as stackyards and stables, and loss of abandoned farm buildings suitable for nest-sites. Organochlorine pesticides in 1950’s and 1960’s and rodenticides in 1970’s and 1980’s had disastrous effects on many owl populations in Europe, particularly NW Europe, and parts of North America; in NE Australia, rodenticides used since 1992 for rat control in cane fields have caused local population declines of 75-85%. Conservation efforts have also included protection and re-establishment of rough-grassland habitat mosaics, providing prey-rich foraging areas, and controls over use of second-generation anticoagulant rodenticides; reintroduction schemes in some areas have had mixed success, as well as conflicting with wild populations. While mortality rates in temperate regions can be very high, birds start breeding in first year, lay large clutches, young grow rapidly, and losses can quickly be replaced, so long as suitable habitat, prey abundance and nest-sites exist. Most die within 1-2 years, but ringing returns have demonstrated that small numbers survive for up to 21 years.
Bibliography. Ali & Ripley (1981), Andrews (1981), Baudvin (1978, 1986), Beckon (1989), Bellocq (1998), Bellocq & Kravetz (1993), Bendel & Therres (1993), Bregulla (1992), Brown (1981), de Bruijn (1979, 1994), Buden (1974), Bühler (1964), Bühler & Epple (1980), Bunn (1972, 1974), Bunn & Warburton (1977), Bunn et al. (1982), Castro & Jaksin (1995), Coates (1985), Colvin (1985), Cottam & Nelson (1937), Cramp (1985), David (1996), Davis, D.H.S. (1959), Derting & Cranford (1989), Dickman, Daly & Connell (1991), Dickman, Predavec & Lynam (1991), Diehl (1988), Downhower (1963), Eckert (1974), Engbring & Ramsey (1989), Epple (1985), Fairley (1966), Fjeldså & Krabbe (1990), Fry et al. (1988), Frylestam (1972), Garrido (1978), Glue (1967, 1974), Goodman (1986), Goodman et al. (1993a, 1993b), Grant & Bannack (1995), Green (1981), de Groat (1983), Gubanyi et al. (1992), Hagemeijer & Blair (1997), Hamilton & Neill (1981), Handrich et al. (1993a, 1993b), Hanna (1995), Harrison et al. (1997), Haverschmidt (1962), Hazevoet (1995), Heim de Balsac (1965), Henny (1969), Herholdt (1986a, 1993), Herrera & Jaksic (1980), Higgins (1999), Hilty & Brown (1986), Howell & Webb (1995a), Ille (1991), Ingram (1959), Jaksic & Yanez (1979, 1980), Jaksic et al. (1982), Jeserich (1967), Johnsgard (1988), Johnson, P.N. (1994), Johnson, R.W. & Rose (1994), Karpen (1986), Kasparek (1986), Knight & Jackman (1984), Knudsen (1981), Konishi (1993), Lander et al. (1991), Lange (1981), Langgemach & Becker (1997), Langrand (1995), Lenton (1984a, 1984b, 1985), Libois et al. (1983), Lohrl (1965), Luthy et al. (1985), Macdonald & Dean (1984), Maestrelli (1973), Marks & Marti (1984), van Marle & Voous (1988), Marti (1969, 1974, 1988, 1989, 1990a, 1990b, 1992, 1994, 1997), Marti & Wagner (1985), Martín, A. et al. (1985), Martínez (1996), Martínez & López (1995), Massemin, Groscolas & Handrich (1997), Massemin, le Maho & Handrich (1998), Massoia et al. (1990), Mayr (1944), McLaughlin (1994), Mees (1964a), Michelat & Giraudoux (1991, 1992, 1993), Mikkola (1983), Millsap & Millsap (1987), Morton (1975), Morton & Martin (1979), Morton et al. (1977), Motta-Júnior & Talamoni (1996), de Naurois (1982, 1983), Nero (1995), Newton et al. (1991), Otteni et al. (1972), Parker, A.R. (1994), Parker, S.A. (1977), Parkes & Phillips (1978), Payne (1971), Perrin (1982), Phillips (1951), Pickwell (1948), Piechocki (1974), Pokines & Peterhans (1997), Poprach (1995), Potter & Gillespie (1925, 1926), Rahm (1960), Read & Allsop (1994), Reese (1972), Riley (1913), Roberts (1991), Rose (1996), Rothschild & Hartert (1913), Roulin (1996a, 1996b, 1996c, 1998), Rudolph (1978), Ruprecht (1979), de Santis et al. (1994), Sauter (1956), Schaden (1992), Schneider (1977), Schönfeld & Piechocki (1974), Shalter & Schleidt (1977), Sharpe (1876), Sharrock (1976), Shawyer (1987, 1994, 1998), Shirihai (1996), Sick (1993), Siegner (1994), Smal (1987), Smith, C.R. & Richmond (1972), Smith, D.G. et al. (1974), Smith, J.D.B. & Cole (1989), Snow & Perrins (1998), Soucy (1979), Stewart (1952, 1980), Steyn (1984), Taylor (1993, 1994), Tirant (1992), Travaini et al. (1997), Valente (1981), Vaurie (1960c), Voous (1950, 1988), Wallace (1948), Wallick & Barrett (1976), Watling (1982), Webster (1973), Wetmore (1968a), White (1915), Wilson, R.T. (1987), Wilson, R.T. et al. (1986, 1987), Wilson, V.J. (1970), Yalden (1994), Zimmerman et al. (1996).