Which bird is the only living species in the family Opisthocomidae? The answer is the hoatzin.
The hoatzin, Opisthocomus hoazin, is the sole living species in the bird family Opisthocomidae. It inhabits swamp forests, river margins, and mangroves in the Amazon and Orinoco basins of South America. The family is also the only named family in the order Opisthocomiformes, although its exact evolutionary placement has long challenged ornithologists.
Hoatzins are especially unusual because they feed heavily on leaves. Their enlarged digestive system ferments vegetation in a way that resembles the foregut fermentation of some mammals, giving them a distinctive reputation as “stinkbirds.” Adults have a tall crest, blue facial skin, and reddish-brown wings.
Their chicks are even more distinctive: they possess claws on two wing digits, which help them climb through branches and escape into water when threatened. Those claws disappear as the birds mature. Fossil relatives show that the hoatzin lineage was once more diverse than the single living species.