Hoatzin

Identification

The hoatzin is pheasant-sized, with a total length of 65 cm (26 in), and a long neck and small head. It has an unfeathered blue face with maroon eyes, and its head is topped by a spiky, rufous crest. The long, sooty-brown tail is bronze-green tipped with a broad whitish or buff band at the end. The upper parts are dark, sooty brown-edged buff on the wing coverts, and streaked buff on the mantle and nape. The underparts are buff, while the crissum (the undertail coverts surrounding the cloaca), primaries, underwing coverts, and flanks are rich rufous-chestnut, but this is mainly visible when the hoatzin opens its wings.<p>It is a noisy bird, and makes a variety of hoarse calls, including groans, croaks, hisses, and grunts. These calls are often associated with body movements, such as wing spreading.<p>Hoatzin chicks have two claws on each wing. Immediately after hatching, they can use these claws, and their oversized feet, to scramble around the tree branches without falling into the water. When predators such as the great black hawk attack a hoatzin nesting colony, the adults fly noisily about, trying to divert the predator's attention, while the chicks move away from the nest and hide among the thickets. If discovered, however, they drop into the water and swim under the surface to escape, then later use their clawed wings to climb back to the safety of the nest. This has inevitably led to comparisons to the fossil bird Archaeopteryx, but the characteristic is rather an autapomorphy, possibly caused by an atavism toward the dinosaurian finger claws, whose developmental genetics ("blueprint") presumably is still in the avian genome. Since 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. Modern researchers, however, hypothesize that the young hoatzin's claws are of more recent origin, and may be a secondary adaptation from its frequent need to leave the nest and climb about in dense vines and trees well before it can fly. A similar trait is seen in turacos, whose nestlings use claws on their wings to climb in trees.

Taxonomy

Scientific name

Opisthocomus hoazin

Authority

(Müller, PLS, 1776)

Order

Opisthocomiformes

Family

Opisthocomidae

Genus

Opisthocomus

Range and habitat

Migration

Resident species

Behavior

The hoatzin is a folivore—it eats the leaves (and to a lesser degree, the fruits and flowers) of the plants that grow in its marshy and riverine habitat. It clambers around along the branches in its search for food. The hoatzin uses a leathery “bump” on the bottom of its crop to help balance its weight on the branches. The species was once thought to eat the leaves of only arums and mangroves, but the species is now known to consume the leaves of more than 50 botanical species. One study, undertaken in Venezuela, found that the hoatzin's diet was 82% leaves, 10% flowers, and 8% fruit. Any feeding on insects or other animal matter is purely opportunistic or accidental.<p>One of this species' many peculiarities is its unique digestive system, which contains specialized bacteria in the front part of the gut that break-down and ferment the foliar material they consume (much like cattle and other ruminants do). This process is more efficient than what has been measured in many other species of birds, with up to 70% of the plant fiber being digested. Unlike ruminants, however, which possess a rumen (a specialized, chambered stomach for bacterial fermentation), the hoatzin has an unusually large crop that is folded into two chambers, with a large, multi-chambered lower esophagus. <p>Serrations on the beak help cut leaves into smaller pieces before they are swallowed. Because they lack the teeth of mammals, hoatzins don't regurgitate their food, or chew the cud; instead, a combination of muscular pressure and abrasion by a “cornified” lining of the crop is used as an equivalent to remastication, allowing fermentation and trituration to occur at the same site. The fermented foliage produces methane which the bird expels through burping. Its stomach chamber and gizzard are much smaller than in other birds. Its crop is so large as to displace the flight muscles and keel of the sternum, much to the detriment of its flight capacity. The crop is supported by a thickened skin callus on the tip of the sternum, which helps the bird support the crop on a branch during rest and while digesting its food. A hoatzin's meal takes up to 45 hours to pass through its body. With a body weight as low as 700 grams (1.5 lb), the adult hoatzin is the smallest known animal with foregut fermentation (the lower limit for mammals is about 3 kilograms or 6.6 pounds).<p>Because of aromatic compounds in the leaves they consume, and the bacterial fermentation required to digest them, the birds have a disagreeable, manure-like odor and are only hunted by humans for food in times of dire need; local people also call it the "stinkbird" because of it. Much of the hoatzin’s diet, including various types of Monstera, Philodendron and other aroids, contains a high concentration of calcium oxalate crystals, which, even in small amounts, can be greatly uncomfortable (and even dangerous) for humans to consume.<p>Hoatzins are seasonal breeders, breeding during the rainy season, the exact timing of which varies across their range. Hoatzins are gregarious and nest in small colonies, laying two or three eggs in a stick nest in a tree hanging over water in seasonally flooded forests. The chicks are fed on regurgitated fermented food.

Conservation

Population trend

declining