Opisthocomus

The hoatzin (Opisthocomus hoazin), also known as the reptile bird, skunk bird, stinkbird, or Canje pheasant, is a species of tropical bird found in swamps, riparian forests, and mangroves of the Amazon and the Orinoco basins in South America. It is notable for having chicks that have 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), 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), and 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 under parts are buff, while the crissum (the undertail coverts surrounding the cloaca), primaries, underwing coverts and flanks are rich rufous-chestnut, but this is mainly visible when it opens its wings. The hoatzin is an herbivore, 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. The alternative name of 'stinkbird' is derived from the bird's foul odour, which is caused by the fermentation of food in its digestive system. 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. The hoatzin was originally described by German zoologist Statius Müller in 1776. 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. A whole genome sequencing study published in 2012 places the hoatzin as the sister taxon of a clade composed of Gruiformes (cranes) and Charadriiformes (plovers). In 2015, genetic research indicated that the hoatzin is the last surviving member of a bird line that branched off in its own direction 64 million years ago, shortly after the extinction event that killed the non-avian dinosaurs. Historical placement as gamebirds is based mainly on phenetic considerations of external morphology, which are considered unreliable and generally dismissed today; the gamebirds together with the waterfowl are classified as Galloanserae whereas the hoatzin are not. However, cuckoos have zygodactyl feet (two toes forward, two backward) and turacos are semi-zygodactylous, whereas the hoatzin has the more typical anisodactyl foot with three toes forward, one backwards. The evolution of avian dactyly, on the other hand, is not entirely resolved to satisfaction. Sibley and Ahlquist in 1990 considered it likely to be a basal cuckoo based on DNA-DNA hybridization. Avise et al. in 1994 found mtDNA cytochrome b sequence data to agree with Sibley and Ahlquist's previous treatment. Subsequently, Hughes and Baker in 1999 proclaimed to have 'resolved' the relationships of the hoatzin to be with turacos, based on their own analysis of 6 sets of mtDNA and one of nDNA sequences.