Hoatzin

The Hoatzin is one of the most unusual and evolutionarily distinct birds in the world, found in swamp forest and riverine vegetation across the Amazon and Orinoco basins of South America. Its odd appearance, awkward flight, and unique biology have long fascinated ornithologists, and its exact evolutionary relationships remain a subject of scientific study.

Perhaps most famously, Hoatzin chicks possess small claws on their wings that allow them to clamber through branches before they can fly, a feature reminiscent of ancient bird ancestors and lost as the chicks mature.

• Crest is a spiky, loose tuft of rufous-buff feathers on the crown, often raised when alert or agitated
• Upperparts are brown, streaked with buff and cream, giving a mottled appearance
• Wings are chestnut-brown, broad and rounded, built for short awkward flights rather than sustained flight
• Underparts are buffy, with a long, broad tail tipped in buff-cream
• Bare facial skin is bright blue, surrounding a red eye, a feature that (along with the crest) makes the head distinctive apart from feather pattern
• The loose, soft body feathers reflect the species' weak flight muscles, distinguishing it structurally from typical gamebird-like species it superficially resembles

Adults of both sexes look alike, with a spiky rufous-buff crest, brown-and-buff streaked upperparts, chestnut wings, and buffy underparts present year-round. Juveniles are duller and browner, notably possessing small claws on the bend of each wing that aid climbing through branches, a feature that disappears as the birds mature and gain flight ability. There is no significant seasonal plumage variation in adults.

Hoatzins are found in swamp forest, flooded riverine vegetation, and mangrove-like habitats along slow-moving rivers and lakes throughout the Amazon and Orinoco river basins of South America. The species is non-migratory and highly sedentary, rarely moving far from its home stretch of river or swamp vegetation, and it is dependent on leafy vegetation overhanging water.

This species feeds primarily on leaves, using an unusual digestive system that ferments plant material, an adaptation unique among birds. Hoatzins are weak, clumsy fliers, generally gliding short distances between branches and relying on clambering rather than sustained flight. They nest colonially in branches overhanging water, allowing chicks to escape predators by dropping into the water and swimming, then climbing back up using their wing claws. Calls include low grunts, hisses, and croaks, and colonies can be noisy and active.