hoatzin (wätsēnˈ) [key] [Aztec], common name for a peculiar marsh bird, Opisthocomus hoatzin. The hoatzin is a slender bird with a brownish plumage spotted with white above and reddish-yellow to rust below. It may reach up to 25 in. (64 cm) in length, but weighs no more than 13/4 lb (810 grams). It has a long tail of 10 loosely bound feathers, and a large, bristly crest mounted on a tiny skull. Its young are good swimmers and are peculiar in having claws on their first and second wing digits, which they use along with their short curved bills and weak toes for climbing trees. In this respect the hoatzin is reminiscent of the extinct lizard-bird Archaeopteryx. As the young mature and begin to fly (though never especially well), the claws dwindle. Hoatzins are sometimes called reptile-birds because of their crocodilian odor and harsh, monotonous call. In yet another respect, they are the most advanced of avians. In other birds, food is broken up in the gizzard, but the hoatzin accomplishes this in its well-developed, muscular, horny-walled crop, and its gizzard is much reduced. The hoatzin's specialized diet consists of certain marsh plants, including the mangrove, and the bird is thus restricted to the riverine forests centering around the Amazon Basin where it lives in small colonies of 10 to 50 birds. Both sexes participate in the building of loosely entwined stick nests, 5 to 20 ft (1.5–6.1 m) over the water, in the forks of riverbank trees. The female lays two to four small eggs per clutch, which are yellowish in color with pink or brown spots. Little is known of the incubation period or of parental responsibilities. Hoatzins are classified in the phylum Chordata, subphylum Vertebrata, class Aves, order Galliformes, family Opisthocomidae.