This order uniquely comprises a single species, the crested hoatzin (Opisthocomus hoazin).  It is a truly remarkable bird that has long intrigued zoologists and whose morphological and genetic features exclude it from any other group. Previous classifications have placed it in the Galliformes, Cuculiformes and Coliiformes, until it was given its own evolutionary branch. It is arboreal and gregarious and lives in the forests and mangroves of South America. It feeds on plants and fruits and has the unique ability to use bacterial fermentation in its highly developed crop to digest its cellulose-rich food like a cow. This adaptation makes it a completely unique trait among birds. But it is not the only one: juveniles have wing claws that enable them to climb trees, a feature which is completely absent in all other modern birds in this form. Based on molecular data, it is currently considered the sister group to Caprimulgiformes.