Hoatzin Facts

The hoatzin is an obligate folivore, a leaf-eater — a niche almost unheard of among birds, since leaves are hard to digest, low in sugar, and often chemically defended. Up to 82% of its diet is tough green leaves and swamp foliage, with the odd flower or shoot. Its solution is the same one a cow uses: foregut fermentation. A massively enlarged crop houses a community of symbiotic bacteria and other microbes that break down cellulose over many hours, while the bird rests and digests, much as a sloth does on its low-energy diet. The fermentation gives off a strong, manure-like smell that both names the "stinkbird" and makes it an unappealing target for hunters.

The hoatzin's most famous trait belongs to its chicks: two functional, reptile-like claws on the leading edge of each wing. If a predator such as a snake or monkey threatens the nest, a chick can drop into the water below and swim away, then use those claws to grip branches and climb back up once the danger passes. Early naturalists took the claws as a primitive link to the dinosaur-era Archaeopteryx, but genetics tells a different story: the hoatzin's line diverged from other birds around 64 million years ago, and the wing claws are not an ancient leftover but a trait re-evolved to fit this specific waterside life. The claws are lost as the bird matures.

The hoatzin is a large, pheasant-sized bird, 24 to 26 inches long and under 2.2 pounds, with a spiky reddish crest, bare blue facial skin, and red eyes. Its oversized fermentation crop comes at a cost: it crowds the flight muscles and shrinks the keel bone they anchor to, leaving the hoatzin a clumsy, reluctant flier that mostly hops and clambers through branches. When it must cross a river it manages only short, laboured glides, often crash-landing in the foliage on the far side.

Hoatzins are confined to the Amazon basin and Orinoco delta of northern and central South America, across Brazil, Peru, Colombia, Ecuador, Venezuela, Bolivia, and Guyana, where it is the national bird. Their habitat is narrow and specific: the tangled riparian vegetation lining slow rivers, oxbow lakes, and seasonally flooded forest, as well as coastal mangroves. Dense growth overhanging deep water gives them both an endless leaf supply and, for their chicks, the water below as an escape route.

Hoatzins are gregarious, living in colonies of 10 to 50 that roost packed together in riverside foliage. Because so much of their time goes to sedentary digestion, they are generally slow and sluggish — but far from quiet, keeping up a harsh chorus of groans, croaks, hisses, and grunts more frog-like than bird-like to hold the flock together and defend territory. Breeding is often cooperative: a dominant pair nests with "helpers," usually their own grown offspring, who defend the territory and help feed and guard the chicks, raising the young's chances of survival.

Breeding is tied to the rainy season, when floods secure the habitat. The group builds a loose stick platform on a branch directly over the water, usually 6 to 15 feet up. The female lays 2 to 3 eggs, incubated by the pair and their helpers for about 28 to 32 days. Hatching sets up the hoatzin's signature defence: a threatened chick drops into the river, swims clear, and later climbs back with its wing claws — a lifeline until it fledges and the claws disappear. Wild lifespan is poorly documented, and the species is hard to keep in captivity because of its specialised diet and digestion.

The hoatzin is Least Concern with a broadly stable population across its huge Amazonian range, helped by living in large, remote areas and by a smell that discourages hunting. But that stability rests entirely on its narrow habitat. The hoatzin is very sensitive to habitat loss, and the clearing of Amazon forest for logging, farming, and ranching is a looming threat; because it needs intact flooded riparian corridors, river damming or mangrove clearance can wipe out local populations quickly. It is, in effect, an indicator of the health of the flooded forests it depends on.

• Cornell Lab of Ornithology - Hoatzin Biology
• National Geographic - The Bizarre Hoatzin
• IUCN Red List of Threatened Species - Opisthocomus hoazin
• Audubon Society - Guide to the Hoatzin