Hoatzin

The hoatzin (pronounced WAHT-sin, HO-tsin, or hoh-AT-sin) is a tropical bird that lives in the swamps, river-edge forests, and mangroves of South America’s Amazon and Orinoco Basins. It is the only living species in its genus, *Opisthocomus*, which is itself the sole living genus in the family Opisthocomidae, under the order Opisthocomiformes. Despite much debate among experts, the family’s exact place on the bird family tree remains unsettled. The species is best known for two unusual traits: its chicks have primitive claws on two of their wing digits, and its digestive system relies heavily on fermentation to break down plant material—a feature more typical of hoofed ruminants and some primates. The hoatzin is also the national bird of Guyana, where it is called the Canje pheasant.

About the size of a pheasant, the hoatzin reaches 65 cm (26 in) in total length, with a long neck and a small head. Its face is bare and blue, its eyes are maroon, and a spiky, reddish-brown crest sits on top of its head. The long, sooty-brown tail has a bronze-green sheen and ends in a broad whitish or buff band. The upperparts are dark, sooty brown with buff edges on the wing coverts and buff streaks on the mantle and nape. The underparts are buff, while the undertail coverts, primaries, underwing coverts, and flanks are a rich reddish-chestnut—mostly visible only when the bird spreads its wings. It is a noisy bird, producing hoarse calls such as groans, croaks, hisses, and grunts, often accompanied by wing-spreading movements.

Hoatzin chicks hatch with two claws on each wing. Right after hatching, they use these claws and their oversized feet to scramble through tree branches without falling into the water. When a predator like the great black hawk attacks a nesting colony, the adults fly about noisily to distract it, while the chicks move away from the nest and hide in thickets. If discovered, they drop into the water, swim underwater to escape, and later use their clawed wings to climb back to the nest. This behavior has drawn comparisons to the fossil bird *Archaeopteryx*, but the claws are considered an autapomorphy—possibly an atavism toward dinosaurian finger claws, whose genetic blueprint may still exist in the bird genome. Wing claws are not unique to hoatzins; chickens also have them. Because *Archaeopteryx* had three functional claws on each wing, some earlier researchers thought the hoatzin descended from it, but modern scientists believe the claws are a more recent adaptation, helping chicks climb in dense vines and trees before they can fly. A similar trait appears in turacos, whose nestlings also use wing claws to climb.

The genus name *Opisthocomus* comes from ancient Greek words meaning “long hair behind,” referring to the bird’s large crest. The hoatzin was first described in 1776 by German zoologist Statius Müller. Its relationships with other birds have been hotly debated. Because of its distinctiveness, it has its own family (Opisthocomidae) and order (Opisthocomiformes). Over time, it has been linked with tinamous, gamebirds, rails, bustards, seriemas, sandgrouse, doves, turacos, other cuckoo-like birds, and mousebirds. A 2014 whole-genome study placed the hoatzin as the sister group to a clade containing cranes and plovers. A 2024 genomic study instead placed it as the sister group to the Phaethoquornithes (which includes many aquatic bird orders), with that combined group being sister to flamingos and grebes. Genetic research in 2015 indicated the hoatzin is the last survivor of a bird lineage that split off 64 million years ago, just after the extinction of non-avian dinosaurs. Another 2024 study suggested a Late Cretaceous origin around 70 million years ago, but noted that this early divergence is shared with most living bird orders, so the hoatzin is no more basal than they are.

The only undisputed fossil of a close hoatzin relative is specimen UCMP 42823, a single cranium from the Miocene, found in the upper Magdalena River Valley in Colombia, part of the well-known La Venta fauna. This fossil has been placed in a distinct, less derived genus, *Hoazinoides*, but clearly belongs to the same family as the living species. It differs markedly because the living hoatzin’s skull is much more domed, rounded, and shortened; these autapomorphies were less pronounced in the Miocene bird. Müller discussed these findings in light of the then-favored hypothesis linking hoatzins with gamebirds, a view that had been controversial from the start. He cautioned that *Hoazinoides* by no means establishes a phyletic junction with other gamebirds, and as we now know, by the time of *Hoazinoides*, essentially all modern bird families were already present.