HBW 3 - Family text: Opisthocomidae (Hoatzin)

Family text: 

Class AVES
Order OPISTHOCOMIFORMES
Family OPISTHOCOMIDAE (HOATZIN)

  • Large, gregarious, arboreal birds, with prominent crest, long tail and large wings.
  • 62-70 cm.
  • NE South America.
  • Tropical forested wetlands.
  • 1 genus, 1 species, 1 taxon.
  • No species threatened; none extinct since 1600.   

Systematics

The Hoatzin (Opisthocomus hoazin) is a monotypic species of uncertain affinities. Since it was first described in 1776, there has probably been more controversy as regards its systematic position than that of any other bird species. It was originally named Phasianus hoazin by P. L. S. Müller, implying an affinity with the Galliformes. In an 1840 publication, C. L. Nitzsch linked it with the Musophagidae, a family which is now considered to belong to the Cuculiformes. Over the years it has variously been allied to the Tinamidae, Cracidae, Rallidae, Otididae, Pteroclidae, Columbidae and Coliidae, and recently to the Cariamidae.

Most frequently, however, the Hoatzin has been considered to share some characters with the Galliformes, and it has tended to be placed near Cracidae. Nevertheless, in recent years there has been a strong move to link it with the Cuculiformes, where it is sometimes placed in close association with the anis (Crotophaga) and the peculiar Guira Cuckoo (Guira guira). The problem is that the Hoatzin is so aberrant in both morphology and behaviour that it does not fit satisfactorily alongside any other group of living birds. Apparently, it has gone through a long and independent evolution, retaining some primitive ancestral characteristics, while evolving other, more recent, specialized features.

More conservative systematists retain the Hoatzin in the order Galliformes, based on osteological and immunological data, and possibly also on account of its microscopic feather structure. In contrast, recent evidence from egg-white protein, DNA-DNA hybridization, scleral ossicles (bones of the eye-ring) and behaviour suggest its affinities are with the Cuculiformes.

However, the evidence is equivocal, and the fact that the Hoatzin has the anisodactyl foot of most birds, with a single toe pointing backwards, rather than the zygodactyl foot of the cuckoos, with two toes that can be turned backwards, has been considered a major obstacle to this view. The evidence of feather lice does not appear to help, as most of the Mallophaga genera found on the Hoatzin are unique, while the one that has been found on other species occurs widely on species such as the Green Ibis (Mesembrinibis cayennensis), a coincidence which is assumed to be the result of these species sometimes sharing the same habitats.

Thus, the nearest living relatives of the Hoatzin, normally regarded as one of the most primitive of modern birds, remain a subject of controversy. Currently, most taxonomists award the Hoatzin at least its own suborder, Opisthocomi. However, the lack of consensus as to which are its closest relatives, combined with a fairly generalized agreement that it is quite distinct from all other birds, suggests that the most appropriate course is probably to place it in its own private order, Opisthocomiformes, a view, advocated by E. Stresemann as long ago as 1934, which has come back into favour fairly recently.

A single Tertiary fossil, Hoazinoides magdalenae, dating from the Miocene 18 million years ago, was discovered in Colombia and has been linked with the Cracidae. The fossil site was in the upper Magdalena Valley in the department of Huila, an inter-Andean area beyond the present day distribution of the Hoatzin, although in that period the Andes had not yet been uplifted. Interestingly, this fossil, the only avian fossil from the site, was found in close association with a fossil monkey, Cebupitheca sarmientoi (see Breeding). Another fossil genus, Filholornis, from the Upper Eocene or Lower Oligocene of France, has been linked to both the cracids and the Hoatzin.

Recently, a new fossil family of landbirds, Foratidae, from the Lower Eocene Green River Formation in Wyoming has been erected for Foro panarium, a new genus and new species, based on a nearly complete, associated skeleton. The skull and mandible are most similar to those of the Hoatzin, but the postcranial skeleton shows some similarities to the Musophagidae. Also of interest is a Hoatzin-like fossil fragment which has been reported from Eocene deposits in Argentina.

Since the ancient, reptilian-like, feathered dinosaur Archaeopteryx had three functional claws on each wing, some earlier systematists speculated that the Hoatzin was descended from it, because nestling Hoatzins have two functional claws on each wing (see Breeding). However, modern workers believe that the young Hoatzin's claws are of more recent origin, and may be a secondary adaptation resulting from its frequent need to leave the nest and climb about in dense vines and trees well before it is capable of flight.

Morphological Aspects

The generic name Opisthocomus comes from the Greek, meaning "wearing long hair behind" and refers to the Hoatzin's most distinctive plumage feature, its unusual crest. Much of the time the narrow, 4-8 cm long, somewhat stiffly shafted, mainly rufous crest feathers remain individually erect in a loose manner. This crest, combined with a red iris, prominent eyelashes, and a large bright blue bare orbital and facial area that extends to the bill and around and beyond the ear, gives the Hoatzin a bizarre and somewhat startled appearance. In addition, this large, but small-headed, long-necked and long-tailed bird often crouches and peers out from between thick leaves in a suspicious manner.

The dorsal plumage is mainly bronzy olive with prominent pale buff streaks on the nape and mantle, while the brown wing-coverts are edged with three distinctive white bars. The ten rectrices of the tail are long, dark brown and very broadly tipped buffy white. The Hoatzin's primaries are chestnut, while the secondaries are brown, like the back and the tail. Below, the throat and breast are buffy white, gradually becoming rich chestnut on the thighs and the vent. The short, laterally compressed, black or dark olive-coloured bill is stout, and the strong-looking legs and large feet are also blackish. Sexes are similar, and adults weigh in the region of 700-900 g.

In general, adult Hoatzins have the appearance of a pheasant or, in the Western Hemisphere where there are no native pheasants, a cracid. Indeed, a common name for the Hoatzin in Venezuela is Guacharaca de Agua or "Water Chachalaca", because it seems similar to the more terrestrial and abundant Rufous-vented Chachalaca (Ortalis ruficauda).

Anatomically, the Hoatzin's greatest peculiarity is its unique foregut, which is far larger than its stomach, and in which it processes great quantities of vegetable matter in the manner of a ruminant (see Food and Feeding). The foregut and its contents sometimes make up 25% of the bird's total weight, while its size imposes strict constraints on much of the bird's lifestyle. In order to accommodate this large bulk, the sternum is greatly reduced, with a consequent reduction of flight muscles, which limits the Hoatzin to flying and gliding only in short, heavy bursts. While flight appears weak and awkward, a bird can cover as much as 350 metres in a single flight, though most flying involves much shorter distances. The bird's legs and feet trail below the body during these short, somewhat laboured, flights.

Birds can grasp branches, but they climb and creep about awkwardly in the often dense vegetation of their habitat, and this means of locomotion appears clumsy. Hoatzins perch in a conventional manner, but much of the time is spent in sternal perching, resting on a sternal callous, while lengthy digestion takes place. The callosity is an elliptical horny patch of skin lying over the rear tip of the sternum.

Only young Hoatzins seem able to swim well (see Breeding), and this is exclusively a predator-avoidance tactic. Adult birds found in the water have almost certainly been frightened there, and swimming by adults is generally rare. Although their legs and feet appear well developed and strong, neither adults nor young hop or walk on the ground. Their legs and feet may have evolved for clutching onto leafy branches over water, but they are not zygodactyl as in the Cuculidae (see Systematics).

The Hoatzin's anatomy and restricted locomotion can be attributed to its folivorous diet. Adults, being essentially non-swimmers, poor fliers and weak climbers, are among the most spacially restricted of all birds. Hoatzin social behaviour probably evolved as a result of their morphological limitations.

Because Hoatzins spend their lives in, and climbing through, thick arboreal vegetation, they show an unusual amount of feather wear and breakage. This is especially evident in the severe abrading of the tail, where the wide, buffy white terminal band is often extremely worn or even missing. Moult of adults takes place, as in most birds, following the breeding season, which is the early dry season. In a six month old bird, primary moult progressed outward from the first primary, but secondary moult had two centres, beginning with the tenth and first feathers. Moult on both sides of the tail began almost simultaneously with the outer and inner pairs progressing evenly towards the third pair.

Few birds have a greater reputation than the Hoatzin for an unpleasant odour; in Guyana the species is even called the "Stinking Pheasant". The smell is described as musky or like fresh cow manure, and this smell is thought to be a result of the processing of foliage in the foregut. However, several authors in Venezuela report little or no odour; this might be the result of different species of leaves being eaten there. W. Beebe cooked and ate the flesh of birds he was preparing, and he reported that although tough, the taste was "clean and appetizing as that of a curassow" (Cracidae); he doubted that the persistent odour was connected with the crop contents.

Habitat

Hoatzins are reported to be found up to an altitude of 500 m, but most birds live in the range of about 5 m above sea-level up to about 200 m. They are completely dependent on riparian vegetation in lowland habitats of the Neotropics.

The Hoatzin's range is exclusively east of the Andes, in the drainage basins of the Rivers Amazon and Orinoco, and along the Atlantic coast in Guyana, Surinam and French Guiana, where the species is associated with rivers that flow north and north-east into the ocean. The kind of water within the habitat, be it salt, brackish or fresh, appears to make little difference to the Hoatzin because it lives entirely in trees, vines and bushes, and is thus independent of the different kinds of aquatic foods that each of these water types support. In many areas the species is frequently associated with giant arums, especially those of the genus Montrichardia, which constitute a favourite food. In coastal zones Avicennia mangroves can be similarly important.

The selection of habitat depends entirely on the existence of dense fluvial-associated vegetation, beside and overhanging sluggish rivers, streams, ox-bow lakes, swamps and lagoons. The same thick, often thorny, vegetation provides the birds with all their basic necessities for life, namely food and nesting and roosting sites. Nevertheless, Hoatzins are patchily distributed throughout their range. Often they seem plentiful in one area, but nearby there is a hiatus in seemingly acceptable habitat; perhaps this is connected with their physical limitations.

General Habits

The Hoatzin's restriction to dense trees, vines and bushes that line the waterways of Neotropical lowlands may have evolved because of the bird's evolutionary choice of a vegetarian diet, and its consequent ruminant behaviour. This led to anatomical modifications, in order to accommodate the bulky foregut (see Morphological Aspects), and to slow digestion, both of which features combine to restrict the bird's capability of movement, which in turn has behavioural implications.

Hoatzins are highly social at all times of the year, and at times can be seen in groups of more than 40 birds or more. Two adults will sometimes perch in bodily contact with each other. During breeding, birds occupy small, densely packed, exclusive territories (see Breeding), but in the dry, non-breeding months they live in large, tight groups of as many as 100 individuals or more.

In the past, various authors reported that Hoatzins were colonial, but long-term ringing studies showed that, when breeding, they maintain exclusive, but exceedingly small, territories. Nevertheless, in Peru a non-territorial group of 28 birds was reported to nest colonially in a single tree.

Hoatzins rain-bathe while perching in trees by spreading their wings and erecting their dorsal feathers. They often climb to exposed perches to sun-bathe, turning their backs to the sun and spreading their flight-feathers. Self-preening is frequent, whereas allopreening has not been observed, either between adults, or between adults and their offspring, in spite of the large numbers of ectoparasitic eggs often clearly visible on their faces and feathers. Some of these parasites have been identified only with the Hoatzin host, but no systematic clues, that would suggest relationships to other living bird families, have been found among those parasites so far examined (see Systematics). Head scratching is direct, under the wing. Non-nesting Hoatzins spend about 75% of the day in two forms of roosting behaviour, conventional perching and sternal perching (see Morphological Aspects).

Voice

Hoatzins are noisy, and groups of birds often call in unison, when the combination of their numbers and their curious vocal repertoire can create a great commotion.

They have a large number of vocalizations including hoarse cries, grunts, croaks, growls and hissing. Many of these calls are accompanied by the spreading of the wings and tail. Contact calls, between members of a social unit, are in a series of 3-10 grunts, "oww" or "ohh". In nest defence, birds use a wheezing, raspy nasal hiss, "waaahh", which is a response to predators, humans and low-level territorial intrusions. A high-intensity raspy hiss is used in defence of young, and a gutteral 0·5 second "rrruuh" is used in the social unit, most commonly between individuals of mated pairs. Chicks out of the nest make location calls, and young will beg for food with raspy peeps.

Food and Feeding

Formerly, it was thought that Hoatzins, obligate folivores, ate only the leaves of arums (Araceae) and mangroves (Avicennia), and were thus confined to fluvial areas where these plants grew. This idea arose because early observations of Hoatzins were made in the vegetation beside saline and brackish waters of rivers in the Guianas, near the Atlantic coast.

Nowadays, they are known to eat more than 50 species of plants. In one study in the Venezuelan llanos, they ate 82% green leaves, 10% flowers and 8% fruits. However, their usual diet in this region consists of fewer than a dozen species of plants, while only some four or five plant species compose three quarters of this diet over the year. Hoatzins are especially partial to the leaves of tropical legume trees, many of which contain toxic compounds (see Relationship with Man). Furthermore, they are selective, choosing to eat young leaves, tender shoots and buds, which are higher in water content, as well as being both easier to digest and more nutritious.

Other leaf-eating birds, such as the Ostrich (Struthio camelus) and grouse (Tetraonidae), have hindgut fermentation like horses. In contrast, Hoatzins ferment their vegetable matter in the foregut like cows, sheep, deer and kangaroos. Plant-eaters obtain most nutrients from the cellulose of plant cell walls, but they have to rely on microbes, such as bacteria, protozoans and fungi, which live in the gut, to break down the cellulose into usable sugars by means of fermentation.

The digestive tract of the Hoatzin is one of extreme adaptation. Unlike the case in other birds, the Hoatzin's crop and lower oesophagus are the main digestive organs. In this, the Hoatzin's foregut is similar to that of cows. A thick-walled, muscular crop and lower oesophagus take the place of the cow's rumen, as the site of active fermentation under constant temperature and acidity. The crop and lower oesophagus are large and voluminous, as compared with the proventriculus and gizzard, which are the main digestive organs in more typical birds. The Hoatzin's foregut contains layers of powerful muscles with folded, ridged interiors, lined with tough, horny tissue to break down the food. In addition, there are two constrictions or filters to the flow of coarse vegetable matter, which slow down the passage of food. In many bird species, the period of time for which food is retained is often measured in minutes, but the Hoatzin holds the record. In experiments, liquids were retained for about 18 hours, and solids for 24-48 hours; these retention times are similar to those found in sheep, and are long enough to maintain stable populations of gut bacteria.

The process of the Hoatzin's foregut fermentation is complex and specialized, because it takes the place of chewing and rumination, producing usable volatile fatty acids, before the food passes through all the usual avian digestive organs. Several reasons have been suggested for this, such as the Hoatzin's high food selectively and typical high avian energetic demands, but recent research has shown that the Hoatzin is able to digest a much higher proportion of its food than other plant-eating birds on comparable diets, and indeed that it is similar in performance to ruminants. Current speculation suggests that foregut fermentation also allows bacteria to detoxify noxious chemicals before the food can be absorbed or fed to young, while the microbial synthesis of essential amino acids and vitamins may help to give the bird a more balanced diet.

Nestling and young Hoatzins are fed from the adults' crops on this same regurgitated half-digested foliage, a sticky, greenish, pre-digested mash. It is rich in bacteria, which help to inoculate the young birds' developing crops with the necessary fermenting microbes. Because of the low nutrient value of the Hoatzin's food, the young grow very slowly.

Hoatzins rarely drink, probably because 70% of their leaf diet comprises water, but when they do drink, they tip the bill into water, then tilt the head back and upwards, like most birds. There are two reports of Hoatzins eating foods that were not vegetable, namely small fish and a crab.

The principal times of diurnal foraging are in the early morning and the early evening, and they usually last one or two hours. Birds generally forage within some 50 m of a watercourse. They spend the hot midday hours sternal perching in the shade, digesting the plant matter in their crops. On moonlit nights they typically vocalize, and travel up to 300 m from their defended territories in order to feed. Then all the birds of a social unit, excepting those individuals that are incubating or brooding, forage close together.

Breeding

Breeding takes place during seasonal rains. Throughout much of the Hoatzin's range the climate is strongly seasonal, with six months of rain, four months of drought, and two intermediate months between the seasonal extremes; as the rains begin at quite different times of the year in the regions roughly 5° north and 5° south of the equator, the timing of breeding is better described by seasonal rainfall than by month. Furthermore, because the rainfall is bimodal in the Guianas, where early observations were made, there were conflicting reports of breeding months.

The highly gregarious Hoatzins live and breed in stable social groups of two to eight birds. Two is the most common unit and more than five is rare. The extra birds are subadult and adult helpers, and ringing has shown that nearly all helpers are the young from previous nestings of the pairs they help. A few immigrant, non-breeding helpers were found, and while they lived in the territories and were accepted by territory holders, the immigrants usually helped very little, more often in territorial defence than in rearing the young. Of all first-year birds studied, 90% stayed and helped their parents during subsequent breeding attempts. By the time helpers were four and five years old, the percentage still helping in their natal unit dropped to 20% and under 10% respectively. Most of the long-term helpers were males, because females are the dispersing sex in the Hoatzin, and most of them had left their natal units by the time they were three years old.

Hoatzin helpers take part in all breeding activities except reproductive copulation and egg-laying. In general, it was found that helpers partially emancipated female Hoatzins, and also that they significantly increased reproductive success. In territories with helpers the young were able to leave the nest, which becomes increasingly predation-prone, on average a week earlier than in territories without helpers. Helpers were particularly active in territorial defence, but they also assisted with nest building, incubation and brooding, and fed both nestlings and fledglings. Yet, despite these apparently clear advantages, about 45% of Hoatzin territories did not have helpers.

These social units defend small, well defined, all-purpose territories throughout the breeding season. Breeding males of adjacent territories sometimes engage in aerial battles, flying at each other from 3-5 m apart, and colliding in mid-air breast-to-breast, pecking and clawing at each other. Usually they fall back into the vegetation, locked together, before separating.

The prominent crest is used in social signalling. The male's crest is strongly erected when he defends his territory, at which time he also extends the neck and spreads his wings and tail. In the female, the crest is depressed and neck retracted when inviting copulation.

In the aforementioned study, the defended territories, all adjacent to watercourses, had a mean distance of 42 m along the water, and were defended up to 75 m away from the water's edge. Diurnal activities were usually confined to an area of 1000-3000 m².

Within breeding territories two kinds of copulation take place, display copulation and reproductive copulation. Display copulations constitute a defence strategy. They consist of brief mountings of females by the breeding male, that are directed towards territorial intrusions and displays by neighbouring Hoatzins. These display copulations take place throughout the year and are briefer than reproductive copulations, whereas the post-display vocalizations are louder and last longer than in reproductive copulations. On the other hand, reproductive copulations have been observed only between the breeding pair, not with their helpers; they occur only during the nest building period and shortly before the laying of the eggs.

The nest is a flat, unlined platform, 30-45 cm wide, made of dry twigs and sticks, sometimes so loosely constructed that the eggs can be seen from below. It is similar to the nest of the Green-backed Heron (Butorides striatus), which sometimes breeds nearby. The typical nest, with or without an overhead canopy, is in dense bushes or trees, 2-5 m above water. Successful nests and nest-sites are reused in subsequent years.

The usual clutch consists of 2-4 eggs, which are somewhat variable in shape from elliptical to oval, and which are said to resemble the eggs of rails (Rallidae). They are white, heavily overlaid with small dots and spots of reddish brown and lavender. The mean measurement of 111 eggs in Venezuela was 46·7 x 33·1 mm. Eggs are laid 1·5-2 days apart, and incubation begins with the laying of the second egg, lasting a total of 30-31 days; as with most other aspects of breeding, all members of the group take their turns. In the same study, renesting occurred as many as six times when a nest failed, as long as the rains continued. Successful nesting early in the season for units of 4-6 birds was followed, on rare occasions, by a second nesting.

The semi-nidicolous young hatch within a day of each other in two-egg clutches, but up to four days apart in larger clutches; the adults eat the eggshells. At hatching, the chicks weigh 17-21 g and are of a pale flesh colour, with sparse down, but by the fifth day the skin has changed to dark brown-black, and by the tenth day the nestlings are covered with a thick coat of dark brown down. Their eyes open within a day of hatching, and by the third day they move about in the nest using their wings and feet to help them; the legs and feet are very large in proportion to the size of the chicks. Growth is slow in the young birds, and their flight-feathers are only emerging from the sheaths by days 20-25.

Nestling Hoatzins are brooded continuously for up to three weeks, by both parents and their helpers. As early as their third day, young Hoatzins will jump out of their nests into the water below to avoid predators, although research has shown that these young have a lower chance of subsequent survival; if they are older, five or six days old, their chances of success are greater. Even the youngest birds, when fleeing predators, are able to swim by propelling themselves with alternate kicks of their feet. They can also dive and swim under water, using the wings synchronously. Although chicks can swim six metres or more, as soon as they reach the bushes and trees of their natal patch they climb up using the two claws on each wing, assisted sometimes by arching the neck over twigs and small branches, and by using their toes and bills. In this they are sometimes encouraged by vocalizations from the adults, but even without help, the young birds make peeping sounds that aid their parents and territorial helpers to find them. On one occasion, five birds of a territorial unit spread their wings over a young bird as it made its way into the safety of a dense cover of leaves. Early authors reported that the young find their way back to their nests. However, recent studies have found that the young do not return to the nest, but are cared for, brooded, fed and defended on various branches within the natal territory.

Undisturbed young Hoatzins leave the nest at 2-3 weeks of age, when they are sometimes coaxed away from the nest by the adults. The juvenile birds are still brooded and fed off the nest for up to two months. By days 55-65 these juveniles can fly short distances. The young shed their wing claws between days 70 and 100, but some individuals have been found to regrow them later; eight of 24 adults had non-functional, calloused-over wing claws. The nestlings' claws are small, rounded hooks on digits two and three (see Volume 1, page 37); some authors, using older systems of digit numbering, report that the claws are on digits one and two.

Successful nesting in the Venezuelan llanos was increased by the presence of helpers, but still only 27% of 404 nests were successful. Most failures in this study were due to predation of entire clutches of eggs or of all the chicks by the wedge-capped capuchin monkey (Cebus olivaceus). Other predators were the tayra (Eira barbara) and also ants, which sometimes attacked pipping eggs or live, newly-hatched young. The occasional disappearance of young was probably caused by avian nest predators, such as the Collared Forest-falcon (Micrastur semitorquatus), the Bicoloured Hawk (Accipiter bicolor), the Great Black Hawk (Buteogallus urubitinga), the Crane Hawk (Geranospiza caerulescens) and the Ornate Hawk-eagle (Spizaetus ornatus). Mammalian nest predators in a Venezuelan study area, where 58% of Hoatzin nests failed, and where neither capuchins nor tayras were present, were the common opossum (Didelphis marsupialis), the grison (Galictis vittata), the crab-eating raccoon (Procyon cancrivorus) and the ocelot (Felis pardalis). The survivorship of 60-80% that was recorded for adult Hoatzins is high.

Nestlings and fledglings are fed for several months with partially digested leaf mash, regurgitated from the crops of brooding birds; young birds will put their heads into the open bills of the adults. They beg for food by pecking at the adults' bills and making high-pitched peeps, behaviour that sometimes continues until they are four to five months old. Yet, between one and two weeks of age, nestling Hoatzins begin to sample leaves near their nests, although they do not feed independently until they are 50-70 days old.

Normally, mated pairs are monogamous, though rarely they can be polygamous, but 7% of the breeding units in one study consisted of more than a single breeding pair within a single defended territory. In these unusual circumstances, which generally occurred in units of 6-7 birds, some birds used the same nest for egg-laying and others made two separate concurrent nests in the same territory.

Rarely, Hoatzins attempt breeding as yearlings, although one such male ended up with infertile eggs. About 10% breed at two years old, whereas most start to breed when they are three years old. In the same study, the recorded nest success rate of first-time breeders was comparable or slightly lower than that of the population as a whole. One breeding male lived more than eight years, but long-term studies will probably find much older individuals in a stable population.

Movements

In accordance with their weak flying abilities, Hoatzins are essentially sedentary. This is especially true of those with the most favourable, predator-free territories on riverine islands.

During the dry non-breeding season, the most stressful time for the birds, many Hoatzins in the Venezuelan llanos move short distances. In particular, those with marginal or poor quality breeding territories move to nearby sites, less than 2 km away, as the water dries off and their arboreal habitats pass through a deciduous phase.

Thus, while their preferred habitat is dense gallery forest, they can tolerate the dry season months in isolated patches of woodland, if these are beside permanent water, supporting trees and bushes that do not entirely defoliate. In the following breeding season, the territorial groups return to the same territories that they abandoned when conditions became unfavourable.

The dry season, in conjunction with the local movements it forces, is the time of greatest natural mortality amongst Hoatzins. A logical consequence of this is that drought years have a strong negative effect on their populations.

Relationship with Man

W. Beebe once described the Hoatzin as "tame to an absurd degree". Thus, the large, sedentary Hoatzin, that lives in conspicuous, highly social groups is well known to all people who share its Neotropical lowland habitat. Nevertheless, there are two quite different cultural reactions to Hoatzins which vary through the nine countries that support populations of the species.

The oldest human relationship with the Hoatzin is, of course, that of indigenous tribal people. In Brazil, these people collect the birds' eggs for consumption. They also occasionally eat the birds themselves, and will take adults for their feathers, for medicinal purposes, and again to use as fish bait. In complete contrast to this, in neighbouring Venezuela Hoatzins are generally despised because they are said to give off a repugnant odour (see Morphological Aspects). The bird's odour, or at any rate its reputation for being both smelly and inedible, has no doubt conferred a measure of protection on it in some parts of its range. Tribal people are, however, a declining element throughout South America, as they become integrated into the contemporary cultures of their sovereign states, or simply wiped out.

Very recently there has been renewed scientific interest in the Hoatzin, as microbiologists and botanists are intensively studying the bacteria and enzymes in the bird's foregut. The natural detoxifiers that it contains, if isolated and transferred to domestic grazing species, such as cows and goats, should permit these mammals safely to eat more native kinds of forage that currently remain toxic for them. This could result in important ramifications both for domestic animal husbandry and for the environment.

Status and Conservation

At present, the Hoatzin's natural habitat still covers an immense area of South America, so the total population is probably still large, and the species relatively secure, at least for the time being. However, future developments will require close monitoring.

The most serious threat to the species is the fast and permanent conversion of its habitat into agricultural concerns, most commonly for rice culture. This has already happened in Guyana, Surinam and Venezuela. Plans for the dyking and canalization of rivers on a vast scale in the Venezuelan llanos, if brought to fruition, would seriously affect Hoatzins in that zone. In all nine of the countries with extant Hoatzin populations, there are heavy pressures emanating from many sources that push for the "reclamation" of "useless" frontier lands, in the name of progress, and to accommodate increasing human populations.

The preservation of small, isolated patches of intact habitat in a vast area of agriculture, although carried out with the best of intentions, is most unlikely to be of much benefit to the Hoatzin. It must be stressed that the bird's very limited capability for dispersal, in itself a probable reason for its patchy distribution in undisturbed habitat, means that it is extremely poorly designed for escape from rapid habitat changes, even though suitable unaltered habitat may be available only a few kilometres away.

The Hoatzin is a bird that thrives in its native habitat, if given sufficient protection. A good example can be seen in the Venezuelan llanos on the Masaguaral cattle ranch, which has been maintained as a private nature reserve for the past 30 years. It is here that much of the most recent research on the Hoatzin has taken place. In the early 1970's the Hoatzin population here was limited to about 20 individuals along the small Guárico River, on the eastern edge of the ranch. Since then, this population has expanded to over 200 birds, in part because of constant protection, and also because some Hoatzins lost their former habitat to rice fields now bordering the south-east side of the ranch. Researchers now believe that the available habitat on the east side of this ranch is saturated. In addition, an isolated area of woodland islands, 5 km to the west on the same protected ranch, and adjacent to a permanent water source maintained in the dry season by deep well pumps, has been naturally colonized from an unknown source.

Similarly, at Limoncocha, close by the River Napo in eastern Ecuador, Hoatzins were an increasing population during the 1970's around a small freshwater lake maintained and controlled by a colony of missionaries. However, the missionaries have since left the station, although the consequences for the Hoatzin population remain uncertain at present. Early in the present century, in Guyana, it was noted that Hoatzins readily spread to the margins of newly constructed canals that served agriculture, if these canals were bordered with the vegetation that the Hoatzins preferred. These examples clearly suggest that existing Hoatzin populations could be maintained easily in suitable habitat in national parks, with no effort other than strict control of human activities. Unfortunately, at present there are few parks within the vast continental range of the Hoatzin that offer adequate protection.

Numerous attempts have been made to maintain Hoatzins in captivity. One zoological society sent three expeditions to Guyana in the early 1960's to collect Hoatzins, acclimatize them and bring them back live. They were unsuccessful in keeping Hoatzins alive in captivity, although a single female survived for nearly six months. In 1989, six Hoatzins were imported from Venezuela by the New York Zoological Society. One bird lived for only a year, but the others have now lived more than five years in the Bronx Zoo. These birds laid and then broke several eggs, and no chicks have been hatched yet. What is perhaps more remarkable is that these birds are currently fed on readily available, standard zoological leafy greens and locally grown browse, rather than on any of the exotic, often toxic, plants of their native habitat. It took biologists and nutritionists nearly one year to adapt them gradually to their captive diets. This significant breakthrough suggests the possibility of successful captive breeding in the future.

 

Genus OPISTHOCOMUS Illiger, 1811

Hoatzin
Opisthocomus hoazin

French: Hoazin huppé German: Hoatzin Spanish: Hoazín

Taxonomy.

Phasianus Hoazin P. L. S. Müller, 1776, Cayenne.
Monotypic family of uncertain affinities. Some morphological similarities with Galliformes, especially Cracidae; recent work suggests relationship with Cuculiformes, near Crotophaginae; closest relatives and position in linear hierarchy remain controversial. Monotypic.

Distribution

E of Andes, from Colombia, Venezuela and the Guianas S to Ecuador, Peru, N & C Brazil and Bolivia.

Descriptive notes

62-70 cm; 700-900 g. Unmistakable. Above dark brown with olive reflections and large white streaks; head rufous with long, narrow, erectile crest; below pale buff to rich chestnut on thighs and vent; broad wings chestnut and brown with white bars on coverts; tail long and broad, with wide pale buffy white tip. Iris red; bare facial skin bright blue; legs, feet and smallish bill black. Female similar to male, but crest said to be slightly shorter. Immature resembles adults.

Habitat

Confined to arboreal vegetation bordering lowland waterways in Neotropics, typically alongside streams, rivers and lakes, of fresh, brackish and salt water; often in vicinity of giant arums (Montrichardia, Caladium); in coastal zones, frequently associated with Avicennia mangroves. In dry season, some populations may be forced to occupy less favourable woodland habitats if normal preferred habitat dries out.

Food and Feeding

Near-obligate folivore; the only bird species known with foregut microbial fermentation, similar to that of ruminants; lengthy digestion of large amount of plant matter, sometimes toxic and low in nutritive value, has influenced morphology, locomotion and social behaviour. Around 80% of intake consists of new growth of green leaves and buds, with over 50 species of plants recorded, e.g. Montrichardia, Avicennia, Cecropia and water hyacinth (Eichhornia); a few flowers and fruits taken in season. Feeds in social groups mainly in early morning and near dusk, and also on moonlit nights.

Breeding

Nests in rainy season, dates varying with locality. Monogamous pairs with up to six helpers, mostly their own young of previous season; rarely polygamous; groups defend small, all-purpose territories beside water. Nest placed in dense trees and bushes over water; unlined, open, flat platform of dry sticks. Usually 2 eggs (2-4); incubation 30-31 days, by all members of social group. Chicks have dark brown down; able to escape predators by diving into water and swimming, then clambering back up into vegetation; normally leave nest at 2-3 weeks old, but unable to fly until c. 60 days old; dependent on adults of social unit for food for 2-3 months. Sexual maturity at 2-3 years.

Movements

Highly sedentary; adults do not swim, walk or hop, and have weak flight abilities. During dry season, some social units from suboptimum habitats join others on short migrations of up to 2 km, to arboreal vegetation situated beside permanent water.

Status and Conservation

Not globally threatened. Widespread in suitable habitat, but very patchily distributed; still locally numerous, but little information available on status from most of range. In French Guiana, has declined due to persecution and is now very local; in Brazil, no longer found close to cities. Due to morphological limitations, a consequence of folivorous diet, unable to move far when habitat becomes unsuitable, notably as result of large-scale clearing, e.g. for rice cultivation or hydrological projects.

Bibliography

Amon (1978), Beddard (1889a), Beebe (1909), Beebe et al. (1917), Blake (1977), Bock (1992), Brush (1979), Campbell & Lack (1985), Chubb (1916), Cracraft (1981), Domínguez-Bello, Lovera et al. (1993), Domínguez-Bello, Michelangeli et al. (1994), Domínguez-Bello, Ruiz & Michelangeli (1993), Garrod (1879), Goeldi (1896), Grajal (1991, 1995a, 1995b), Grajal & Parra (1995), Grajal & Strahl (1991), Grajal et al. (1989), Grimmer (1962), Haverschmidt (1968), Heilmann (1926), Hellmayr & Conover (1942), Hilty & Brown (1986), Howard (1950), Lucker (1987), Meyer de Schauensee & Phelps (1978), Miller (1953), Morell (1994), Olson (1992), Ortiz & Carrión (1991), Pinto (1964), Pycraft (1895), de Queiroz & Good (1988), Quelch (1888, 1890), Ramo & Busto (1984), Ruiz et al. (1994), Ruschi (1979), Rutgers & Norris (1970), Rutschke (1970), Sears (1994), Sibley (1994), Sibley & Ahlquist (1972, 1973, 1990), Sibley et al. (1988), Sick (1985a, 1985c, 1993), Skutch (1935, 1966), Snyder (1966), Stegmann (1978), Strahl (1985, 1988), Strahl & Schmitz (1990), Stresemann & Stresemann (1966), Thiollay (1988), Thomas (1979), Torres (1987), Tostain et al. (1992), Van Tyne & Berger (1959), Vanderwerf & Strahl (1990), Webb (1965), Young (1929).