primitive chicken-sized bird of South American swamps, principally in the Amazon and Orinoco river basins. The young possess two large claws on each wing, a trait that has led some scientists to link the species with the fossil Archaeopteryx of the dinosaur era. The hoatzin is the only bird with a digestive system that ferments vegetation as a cow does, which enables it to eat leaves and buds exclusively. Hoatzins feed on swamp plants, grinding foliage in a greatly enlarged crop (not the gizzard, as in other birds). Adults can fly clumsily for short distances, but they spend most of their time perched, digesting their leafy food. A large rubbery callus on the bird's breastbone acts as a tripod to keep it from falling over when its stomach is distended.

The hoatzin was first described scientifically in 1776, but molecular biology techniques were required to confirm it as a relative of the cuckoo (order Cuculiformes (cuculiform)). Fossil evidence from France suggests hoatzins may have lived over 36 million years ago, during the Eocene Epoch. Hoatzins have existed in Colombia since the Miocene Epoch, which began over 20 million years ago.