hoatzin, (Opisthocomus hoazin), 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 is about 65 cm (25.6 inches) long but weighs less than 1 kg (2.2 pounds). It has a long tail, plumage streaked brown above and yellowish below, a loose head crest, and a blue face with bright red eyes. Sexes look alike, and both parents, as well as older siblings, cooperate to raise two to five young. After four weeks of incubation, the eggs hatch, and adults feed the chicks a leaf paste regurgitated from the crop. Adult hoatzins hiss, hoot, and yelp at predators, such as tayras and capuchin monkeys. Nests are built over water, and if danger threatens, the young, which are excellent swimmers, will plunge to safety, return to shore, and use their claws to climb back up to the nest.
The hoatzin was first described scientifically in 1776 and has been associated with several bird orders at various times since its discovery. From its external features, it has been linked previously to fowl-like birds of the order Galliformes. Although many authorities presently classify the hoatzin with the cuckoos in the order Cuculiformes, the hoatzin’s foot structure differs from that of other members of the order. (Hoatzins have feet with three toes forward and one behind, whereas cuckoos have feet with two toes forward and two behind.) The hoatzin’s foot morphology combined with its unique internal features have prompted some scientists to assign the hoatzin to its own group, order Opisthocomiformes. 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.