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The Hoatzin (Opisthocomus hoazin), also known as the Hoactzin, Stinkbird, or Canje Pheasant, is a species of tropical bird found in swamps, riverine forest and mangrove of the Amazon and the Orinoco delta in South America. It is notable for having chicks that possess claws on two of their wing digits.

It is the only member of the genus Opisthocomus (Ancient Greek: "wearing long hair behind", referring to its large crest[clarification needed]), which in turn is the only extant genus in the family Opisthocomidae. The taxonomic position of this family has been greatly debated, and is still far from clear.

The Hoatzin is pheasant-sized, with a total length of 65 centimetres (26 in), with a long neck and small head. It has an unfeathered blue face with maroon eyes, and its head is topped by a spiky, rufous crest. The long, sooty-brown tail is broadly tipped buff. The upperparts are dark, sooty-brown-edged buff on the wing coverts, and streaked buff on the mantle and nape. The underparts are buff, while the crissum, primaries, underwing coverts and flanks are rich rufous-chestnut, but this is mainly visible when it opens its wings. The alternative name of "stinkbird" is derived from the bird's manure-like odour, caused by its digestive system. The Hoatzin is herbivorous, eating leaves and fruit, and has an unusual digestive system with an enlarged crop used for fermentation of vegetable matter, in a manner broadly analogous to the digestive system of mammalian ruminants.

This is a noisy species, with a variety of hoarse calls, including groans, croaks, hisses and grunts. These calls are often associated with body movements, such as wing spreading. Calls are used to maintain contact between individuals in groups, warn off threats and intruders and by chicks begging for food.

The Hoatzin was originally described by German zoologist Statius Müller in 1776. It is arguably the most enigmatic living bird in regard to its phylogenetic relationships. No satisfying evolutionary hypothesis has been proposed, and the situation has become worse with the availability of DNA sequence data.

There has been much debate about the Hoatzin's relationships with other birds. Because of its distinctness it has been given its own family, the Opisthocomidae, and its own suborder, the Opisthocomi. At various times, it has been allied with such taxa as the tinamous, the Galliformes (gamebirds), the rails, the bustards, seriemas, sandgrouse, doves, turacos and other Cuculiformes, and mousebirds. Altogether, it has been most frequently suggested to be related to Galliformes, turacos, or the anis (New World cuckoos). [continue reading on Wikipedia.]