Hoatzin
The hoatzin ( ) or hoactzin ( ) (Opisthocomus hoazin) 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 the only extant species in the genus Opisthocomus which is the only extant genus in the Opisthocomidae family under the order of Opisthocomiformes. The taxonomic position of this family has been greatly debated by specialists, and is still far from clear. It is notable for having chicks that have claws on two of their wing digits. Description The hoatzin is pheasant-sized, with a total length of , 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 bronze-green tipped with a broad whitish or buff band at the end. The upper parts 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 (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. It is a noisy bird, and makes a variety of hoarse calls, including groans, croaks, hisses, and grunts. These calls are often associated with body movements, such as wing spreading. Young wing claws Hoatzin chicks have two claws on each wing. Immediately on hatching, they can use these claws, and their oversized feet, to scramble around the tree branches without falling into the water. When predators such as the great black hawk attack a hoatzin nesting colony, the adults fly noisily about, trying to divert the predator's attention, while the chicks move away from the nest and hide among the thickets. If discovered, however, they drop into the water and swim under the surface to escape, then later use their clawed wings to climb back to the safety of the nest. This has inevitably led to comparisons to the fossil bird Archaeopteryx, but the characteristic is rather an autapomorphy, possibly caused by an atavism toward the dinosaurian finger claws, whose developmental genetics ("blueprint") presumably is still in the avian genome. Since Archaeopteryx had three functional claws on each wing, some earlier systematists speculated that the hoatzin was descended from it, because nestling hoatzins have two functional claws on each wing. Modern researchers, however, hypothesize that the young hoatzin's claws are of more recent origin, and may be a secondary adaptation from its frequent need to leave the nest and climb about in dense vines and trees well before it can fly. Taxonomy, systematics, and evolution The generic name Opisthocomus comes from Ancient Greek ópisthokomos derived from ópisthe ( ópisthen before a consonant) "behind" and kómē "hair" altogether meaning "long hair behind" referring to its large crest. The hoatzin was originally described in 1776 by German zoologist Statius Müller. Much debate has occurred about the hoatzin's relationships with other birds. Because of its distinctness, it