The hoatzin is the only representative of the family Opisthocomidae. The alternative name of “stinkbird” is derived from the bird’s manure-like odor caused by its digestive system. The hoatzin is an herbivore and has an unusual digestive system with an enlarged crop used for fermentation of vegetable matter. The digestive system is analogous to the digestive system of mammalian ruminants. The hoatzin 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. At various times, it has been allied with such taxa as the tinamous, the Galliformes (gamebirds), the rails, the bustards, seriemas, sandgrouse, doves, turacos , Cuculiformes, and mousebirds. Altogether, it has been most frequently suggested to be related to Galliformes, turacos, or the anis (New World cuckoos).