Opisthocomiformes is a tiny order of birds containing only one species, Opisthocomidae or, as commonly called, hoatzin. The hoatzin are found in South American swamps, primarily the Amazon and the Oronoco areas. Hoatzin are the only bird known that ferments its food in the stomach, similarly to the way a cow digests its food. A large, rubbery tripod appendage called the callus sticks out on the bird?s breastbone to keep it from tipping over when its stomach is distended during digestion.