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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.