Opisthocomiformes

Hoatzins

(Opisthocomidae)

Class Aves

Order Opisthocomiformes

Family Opisthocomidae

Number of families 1

Thumbnail description
Medium-sized, crested birds with bare face and long tail; obligate herbivore with foregut microbial fermentation

Size
24.5–27.5 in (62–70 cm); 1.4–1.9 lb (650–850g)

Number of genera, species
1 genus; 1 species

Habitat
Riparian arboreal vegetation along rivers and streams, mangrove swamps

Conservation status
Not threatened

Distribution
East of the Andes in the Orinoco and Amazon river basins and in the Guianas

Evolution and systematics

The Opisthocomidae is a monotypic family that is restricted to tropical South America. Its sole representative, the hoatzin (Opisthocomus hoazin), is unique in appearance, behavior, morphological specializations, and physiological adaptations, and it is considered one of the most primitive of existing birds.

The hoatzin's phylogenetic relationship to other birds is uncertain and has been strongly debated since it was first described in 1776 (as Phasianus hoazin by P.L.S. Müller). Hoatzins were originally considered allied to the galliforms (fowl-like birds), and because of their chachalaca-like appearance hoatzins were placed near the Cracidae (chachalacas, guans). On the basis of osteology, mitochondrial and nuclear gene sequences, protein electrophoresis, and DNA-DNA hybridization, hoatzins have also been considered a cuculiform or a sister group to the cuculiforms, allied with the Neotropical anis (Crotophagidae) and the Guira cuckoo (Guira guira). However, hoatzins have an anisodactyl foot (a single toe pointing backwards), unlike the zygodactyl foot (two toes pointing backwards) of all cuckoos.

A more comprehensive comparative study of mitochondrial and gene sequences strongly supports placing hoatzins in the basal portion of a clade with turacos (Musophagidae) and not among or basal to cuckoos. Comparative studies of pterylosis (feather tract patterns), osteology, and growth pattern of primary wing feathers further support a sister relationship between hoatzins and turacos. However, until the relationship is better defined it is prudent to place hoatzins in their own order.

The relationship between hoatzins and turacos (the latter are restricted to sub-Saharan Africa) poses some interesting zoogeographic problems because the geographical distributions of these two groups are widely separated. Nevertheless, a fossil (Foro panarium) from the lower Eocene Green River formation of Wyoming in the United States shares osteological similarities to hoatzins and turacos. The only fossil of a hoatzin is Hoazinoides magdalenae from the Miocene of Colombia.

Physical characteristics

Hoatzins are medium-sized birds with a long tail, long neck, and small head. The bright blue face is bare, the iris is red, and the eyelashes are prominent. The head is adorned with a fan-shaped shaggy crest that gives hoatzins an unusual and unmistakable appearance. Upperparts are mostly bronzy-olive streaked with buff, the chest is buff, the underparts are mostly chestnut, and the tail tip is buff. Sexes are similar in appearance and juveniles resemble adults.

Young hoatzins have two well-developed wing claws on digits two and three that are used to clamber about the vegetation when in danger. Researchers originally thought this feature was retained from ancestral Archaeopteryx but now believe that it evolved secondarily.

The sternum, pectoral girdle, and associated flight musculature of hoatzins are uniquely modified to accommodate a very large crop that functions as a fermentation chamber.

Distribution

Hoatzins are endemic to South America and are patchily distributed east of the Andes from Venezuela and the Guianas, southwards to Colombia, Ecuador, Peru, Brazil, and Bolivia. Hoatzins live along rivers and streams in the Orinoco and Amazon basins and on the Atlantic coast of the Guianas.

Habitat

Hoatzins are never found far from water. They are restricted to riparian arboreal vegetation bordering rivers, streams, lagoons, swamps, and oxbow lakes of the Amazon and Orinoco river systems. Along the Atlantic coast of the Guianas hoatzins frequently inhabit mangrove swamps. Hoatzins are limited to warmer lowlands from sea level to about 1,640 ft (500 m).

Behavior

In the central plains (Llanos) of Venezuela, hoatzins are territorial, colonial breeders. A monogamous breeding pair of adults and up to five nonbreeding helpers (retained from the previous year's nesting attempt) form the social unit. Male and female helpers seem to be equally common and both sexes participate in all reproductive activities except egg laying.

Territories are established throughout the breeding season (the rainy season) along waterways, and all activities are carried out within these multipurpose territories. When not tending a nest, hoatzins spend most daylight hours (70–80%) sitting or resting on a thickened callous of skin on the posterior tip of the sternum. Most of the rest of their time is spent foraging or in territorial disputes.

All members of the social unit (breeders and nonbreeders) defend territories against neighboring groups. Display copulations are a common form of territorial behavior and occur in response to intruders or similar displays by neighboring birds. Other territorial defense behaviors include postures, territorial vocalizations, chasing of intruders, and aerial battles. Maintenance behavior includes bathing during rainstorms and preening (but not allopreening, or mutual preening).

During the nonbreeding season most birds abandon their territories and form temporary flocks of up to 100 individuals. However, territories where reproductive success is high are defended year-round. Hoatzin groups are noisy; their hoarse shrieks, hisses, grunts, and growling sounds can be heard from afar. Adults are agile climbers but rather weak flyers; nevertheless, they are capable of flying up to 380 yd (350m) without rest.

Feeding ecology and diet

Hoatzins are obligate herbivores, feeding almost exclusively on tree leaves. They prefer young leaves and shoots but also eat flowers and buds. In Venezuela, the diet of hoatzins includes the leaves of trees and shrubs belonging to families such as Fabaceae, Polygonaceae, Sterculiaceae, Combretaceae, Rutaceae, Vitaceae, and Lecythidaceae. Hoatzins have regularly spaced foraging bouts throughout the day; major bouts occur near sunrise and sunset, they also forage on moonlit nights.

Hoatzins are unique among birds in having a well-developed microbial fermentation system in the crop and caudal esophagus. In all other birds known to ferment plant material, microbial activity takes place in the hindgut. In hoatzins, the crop is greatly enlarged, thick walled, and muscular and is also the main site for leaf grinding. The microflora of the crop is composed of mixed bacterial populations and ciliate protozoa. Hoatzins retain the digesta for long periods of time, exposing leaves to bacterial enzyme attack. The end products of bacterial fermentation are volatile fatty acids that appear to be absorbed through the crop wall and used by the bird as energy substrates. Crop bacteria are likely to become a source of protein, carbon, and other nutrients because hoatzins have enzymes in the stomach that display bacteriolytic activity. Partially digested plant material, bacteria, and protozoa flow

through the stomach and their constituents can then be absorbed through the intestinal wall.

The crop may also function as a detoxification chamber. Leaves of the plants eaten by hoatzins contain a broad array of phytotoxins, and it is likely that microorganisms in the crop degrade dietary toxins. Crop bacteria are implicated in the detoxification of saponins, a toxic compound present in several of the plants eaten by hoatzins in Venezuela. Hoatzins avoid highly tanniferous leaves and cannot handle tannin-rich diets.

Reproductive biology

Hoatzin parents and nonbreeders build the nest, which is an unlined platform made of twigs on branches 6.5–16.5 ft (2–5 m) high that are situated directly over the water. Courtship behavior is not very conspicuous, but reproductive copulation by breeders is common within several days of egg laying. Reproductive copulations are longer in duration than are display copulations. The clutch has one to six eggs, but two eggs are the most common; clutches of four to six eggs seem to be the result of two females laying in the same nest. Eggs are laid 1.5–2 days apart and hatch after 30–31 days of incubation. Incubation begins with the second egg when more than one egg is laid.

The young weigh 0.6–0.7 oz (17–21 g) at hatching and are almost naked. The crop of hoatzin chicks is sterile; the crop is inoculated during a chick's first two weeks of life through feeding by adults. At day 20 chicks are covered with down and adult-like feathers begin to grow. Nestlings are brooded almost continuously for up to three weeks by all members of the social unit. If the nest is not disturbed, the young leave the nest at two to three weeks of age (3.9–5.6 oz; 110–160 g). However, four- to six-day-old chicks will plunge into the water when approached by a predator. At this age, they are capable of swimming underwater and can climb through the vegetation using their claws, wings, bill, and feet. The young will not return to the nest once disturbed. Young are brooded and fed off the nest for one or two months after nest departure. They are able to fly sometime between days 55 and 65 (12–16 oz; 350–450 g). Wing claws are shed at 70–100 days.

Hoatzins are seasonal breeders and reproductive activity coincides with the rainy season. In the Venezuelan Llanos about 47% of social units raise at least one young to independence per season. Nest failure is almost entirely attributable to predation, mainly by the wedge-capped Capuchin monkey (Cebus olivaceus).

Conservation status

Hoatzins are locally common but patchily distributed. They are not threatened or endangered but their use of restricted habitats and their specialized diet make them potentially vulnerable. Some watersheds that hoatzins inhabit are being converted to agricultural land and are heavily polluted with runoff pesticides and fertilizers from rice fields. The consequences of this threat are not yet known.

Significance to humans

Hoatzins are not regularly consumed for food because they have a strong musky odor that resembles the smell of cow manure. This strong smell is produced by volatile fatty acids in the crop. The local name given to hoatzins in Guyana (where the it is the national bird) is stinking pheasant, which aptly describes this feature of the bird. However, in Brazil, where hoatzins are called Cigana, locals consume hoatzin eggs and occasionally the birds themselves. Hoatzins have proven difficult to keep in captivity, but the Bronx Zoo in New York has been able to keep a group of birds in a public exhibit for more than ten years.

Resources

Books

del Hoyo, J., A. Elliot, and J. Sargatal, eds. Handbook of the Birds of the World. Vol. 3, Hoatzin to Auks. Barcelona: Lynx Edicions, 1996.

Periodicals

Blair Hedges, S., M.D. Simmons, M.A.M. van Dijk, G-J. Caspers, W.W. de Jong, and C.G. Sibley. "Phylogenetic Relationships of the Hoatzin, an Enigmatic South American Bird." Proceedings of the National Academy of Science 92(1995): 11662–11665.

Domínguez-Bello, M.G., F. Michelangeli, M.C. Ruiz, A. Garcia, and E. Rodriguez. "Ecology of the Folivorous Hoatzin (Opisthocomus hoazin) on the Venezuelan Plains." Auk 111 (1994): 643–651.

Domínguez-Bello, M.G., M.C. Ruiz, and F. Michelangeli. "Evolutionary Significance of Foregut Fermentation in the Hoatzin (Opisthocomus hoazin, Aves: Opisthocomidae)." Journal of Comparative Physiology B 163 (1993): 594–601.

Grajal, A. "Structure and Function of the Digestive Tract of the Hoatzin Opisthocomus hoazin: a Folivorous Bird with Foregut Fermentation." Auk 97 (1995): 20–28.

Grajal, A., S.D. Strahl, R. Parra, M.G. Dominguez, and A. Neher. "Foregut Fermentation in the Hoatzin, a Neotropical Leaf-Eating Bird." Science 245 (1989): 1236–1238.

Hughes, J.M., and A.J. Baker. "Phylogenetic Relationships of the Enigmatic Hoatzin (Opisthocomus hoazin) Resolved using Mitochondrial and Nuclear Gene Sequences." Molecular Biology and Evolution 16 (1999): 1300–1307

Sibley, C.G., and J.E. Ahlquist. "The Relationships of the Hoatzin." Auk 90 (1973): 1–13.

Veron, G., and B.J. Winney. "Phylogenetic Relationships within the Turacos (Musophagidae)." Ibis 142 (2000): 446–456.

Carlos Bosque, PhD