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It is pheasant-sized, but much slimmer, long-tailed, long-necked and has a small head. It has an unfeathered blue face with red eyes, and its head is topped by spiky crest. It is a weak flier.
There has been much debate about its relationships with other bird species. It has been given its own family, the Opisthocomidae, and was previously grouped with the gamebirds. Sibley considered it more likely to be a Cuckoo, although this is disputed because the cuckoos have zygodactyl feet (two toes forward, two backward), whereas the Hoatzin has the more typical three toes forward, one backwards.
The Hoatzin eats the leaves and fruits of the plants which grow in the marshes where it lives. One of this species' many peculiarities is that it has a digestive system unique amongst birds. Hoatzins use bacterial fermentation in the front part of the gut to break down the vegetable material they consume, like cattle and other ruminants.
Hoatzins are gregarious and nest in small colonies, laying 2-3 eggs in a stick nest in a tree overhanging water. The chick, which is fed on regurgitated cud, has another odd feature; it has two claws on each wing, which help it grip branches as it climbs through the trees. This has inevitably led to comparisons to the fossil Archaeopteryx.
This is a noisy species, with a variety of hoarse calls, one of which has been described as like a heavy smoker's wheezing.