Hoatzin

Name and taxonomy

The genus name Opisthocomus is built from two Ancient Greek words, one for behind and one for hair. Together they point at the long crest that stands up on the back of the bird's head. The German zoologist Statius Müller gave the species its first formal description in 1776. Since then it has kept a family of its own, the Opisthocomidae, and an order of its own.

English speakers also call the bird the stinkbird, and Guyanese speakers call it the Canje pheasant. Spanish names vary across the range: chenchena in the Llanos, shansho in Peru, pava hedionda in Colombia. Venezuelans say guacharaca de agua, and Bolivians say pava serere. In Brazil the usual Portuguese name is cigana.

Evolution and relatives

Where the hoatzin belongs in the bird family tree has been argued over for two centuries. Writers have tied it in turn to tinamous, gamebirds, rails, bustards, seriemas, sandgrouse, doves, turacos, cuckoos and mousebirds. A whole-genome study in 2014 made it the sister group of the cranes and the plovers together. A second genomic study in 2024 moved it beside a large group of water birds instead. That combined group came out as the sister of the flamingos and the grebes.

Genetic work published in 2015 dated the split of the hoatzin line to 64 million years ago. That is just after the extinction that removed the non-avian dinosaurs. The 2024 study pushed the origin back to about 70 million years ago, in the Late Cretaceous. It also found that many other bird orders split at the same time, so the hoatzin is no more ancient than they are. The oldest known member of the order is Protoazin from France, about 34 million years old. Relatives also lived in Brazil about 24 to 22 million years ago and in Namibia about 16 million years ago.

Appearance

The hoatzin is about the size of a pheasant, at 61 to 66 centimetres from bill to tail tip. Wikipedia gives a total length of 65 centimetres for the species. Adults weigh from about 0.7 kilograms, and heavier birds reach about 1 kg. The head is small and the neck is long, which makes the body look bulky by comparison. A crest of stiff reddish-brown feathers rises from the crown.

The bare skin of the face is blue, and it sets off a maroon eye. Upperparts are dark sooty brown, with buff edging on the coverts and buff streaks down the nape. The long tail shows bronze-green, and a broad whitish or buff band closes it. Rich chestnut sits on the flanks and under the wings, and it shows only when a bird opens up. Newly hatched chicks are naked, then grow a coat of black down.

Habitat and range

The hoatzin lives in the lowlands of the Amazon and Orinoco basins and in the Guianas. It keeps to swamp, freshwater marsh, gallery forest, mangrove and the wooded banks of rivers, lakes and streams. Seasonally flooded forest suits it best, because it nests over standing water. Birds appear to stay within a short flight of water for their whole lives.

Colombian checklists treat the species as a resident that does not migrate. GBIF holds records from 16 countries, though most of those records come from Colombia, Ecuador and Peru. The bird also reaches Brazil, Bolivia, Venezuela, Guyana, French Guiana and Suriname. iNaturalist carries 4306 observations, which shows how often river tours find it.

Diet and feeding

The hoatzin is a folivore, which means it lives on leaves. One study in Venezuela measured a diet of 82 percent leaves, 10 percent flowers and 8 percent fruit. The species was once thought to take only arum and mangrove leaves. It is now known to eat leaves from more than 50 kinds of plant. Any insect it swallows arrives by accident with the greenery.

Serrations along the bill cut a leaf into pieces before the bird swallows it. The pieces go into a crop that is folded into two chambers and packed with bacteria. Those bacteria break down plant fibre, and up to 70 percent of it is digested. Muscle pressure and a hardened crop lining grind the leaves, in place of the chewing a cow does. A single meal can take 45 hours to pass through the bird. The process releases methane, which the hoatzin burps out.

At a body weight of about 700 grams, this is the smallest animal known to ferment food in its foregut. The lower limit among mammals is roughly 3 kilograms. The crop is so large that it crowds out the flight muscles and the keel of the breastbone. Many of the leaves the bird takes, including aroids, hold crystals of calcium oxalate that would hurt a person.

Behaviour and social life

Hoatzins are sociable and live in flocks, and they nest in small colonies. A breeding pair holds a stretch of waterside and both birds defend it. Good sites over water are scarce, so neighbours press hard on one another. Young birds often remain on their parents' ground for two years or so after hatching. They act as helpers, feeding the next brood and joining in the defence.

The bird clambers along branches on its way to food, and it uses its wings for balance. A leathery pad under the crop rests on a branch and takes some of the weight. Flight is short, heavy and clumsy, because the crop leaves little room for flight muscle. A hoatzin probably spends much of the day sitting still while its crop works. When a great black hawk comes at a colony, the adults fly about noisily to pull it away.

Communication and senses

The hoatzin is a loud bird with a rough voice. Its calls include groans, croaks, hisses and grunts. Birds often pair a call with a movement of the body, such as spreading the wings. A pair marks the edge of a territory with loud noise, aggressive postures and ritual matings.

The large maroon eye and the daytime habits suggest that sight leads the bird to its food. Leaves do not run away, so a hoatzin probably needs less sharp senses than a hunter. The smell of the bird is probably a by-product of fermentation rather than a signal. Chicks beg from the adults and take fermented food straight from the bill.

Reproduction and life cycle

Hoatzins breed once a year, in the rainy season, and the timing shifts across the range. Pairs build a loose platform of sticks on a branch that hangs over water. The female lays two or three eggs. Both parents sit on the clutch, and incubation runs about 32 days. Chicks stay in the nest for 14 to 21 days after they hatch.

Adults feed the chicks on regurgitated leaf pulp that is already fermenting. That meal also passes on the gut bacteria the young bird will need for life. Each chick has two claws on each wing and oversized feet. With them it scrambles about the branches around the nest without falling in. A chick that is found by a predator drops into the water and swims below the surface. It then hauls itself back up the stems with the wing claws.

Young hoatzins can take up to 70 days to fly at all, because the crop is heavy. The claws are lost as the bird matures, and no adult keeps them. Birds do not breed until after their first year. A general bird reference gives about 14 to 15 years in the wild and up to 30 in captivity. None of the four research sources this site uses publishes a hoatzin lifespan, so the page states no measured figure.

Predators and threats

Chicks are the vulnerable stage, and monkeys, snakes and hawks take them from the nest. The great black hawk is the raider named most often in accounts of colonies. Adults answer a raid with noise and movement, which draws the hunter away from the young. A chick that escapes into the water has a real chance of getting back to the nest.

People appear to be a smaller threat here than they are to most large tropical birds. Hoatzin meat has a bad name for its taste, so adults are only taken in hard times. In Brazil some indigenous groups collect the eggs, and the Cuiba of Colombia hunt the bird. The real pressure comes from clearance of Amazonian forest and drainage of inland wetland.

Population and conservation

The IUCN Red List puts the hoatzin in the Least Concern category. The bird is still common over a large part of a very large range. Its numbers are likely falling, because the riverside forest it needs is being cleared. No global count appears to have been published for the species.

Regional lists tell a sharper story in places. The French committee of the IUCN lists the hoatzin as Endangered in French Guiana. Colombian sources treat it as a resident that the national red book has not assessed. Protected wetlands along the Amazon and Orinoco probably hold the strongest colonies.

Relationship with humans

The hoatzin and people appear to have kept a distance from one another for a long time. The smell that follows the bird has kept it off most tables. Egg collecting is local and small, and hunting of adults is rare. River lodges and boat trips now show the bird to visitors, which probably gives it a value alive.

Guyana made the hoatzin its national bird and put it on the coat of arms. The country calls it the Canje pheasant, after the Canje River. No one keeps hoatzins as pets, and the diet of fresh leaves is probably the reason. Zoos appear to have failed with the species for the same reason.

In culture and myth

The bird carries its reputation in its names. In Colombia it is the pava hedionda, the stinking turkey. In Brazil the Portuguese name cigana probably likens the crest and manner to a gypsy. Quechua speakers in Peru know it as shanshu, and Venezuelans as guacharaca de agua.

Guyanese heraldry sets the Canje pheasant at the foot of the national shield. The heraldic text calls it a rare bird found mainly in that part of the world. Spanish writing often calls the hoatzin a living fossil, though the label is not accurate. Early naturalists compared the clawed chicks with Archaeopteryx and guessed at a direct line of descent. Researchers reject that guess, but the picture of a small dinosaur in the reeds seems to have stuck.