Hoatzinidae

Hoatzinidae is a family of birds comprising a single living species, the hoatzin (Opisthocomus hoazin), found in the Amazon and Congo rainforests. This pheasant-sized bird is unique for several reasons, most notably its unusual digestive system. Hoatzins are folivores, meaning they primarily eat leaves. Their digestive process involves bacterial fermentation in a large crop, similar to that of ruminant mammals. This fermentation allows them to break down the cellulose in leaves, but it also produces a strong, musky odor, earning them the nickname "stinkbird." This odor is thought to deter predators.

Another remarkable feature of the hoatzin is its juvenile stage. Hoatzin chicks possess claws on their wings, which they use to climb through the dense vegetation of their arboreal habitat. If threatened, they can drop from branches into water and swim to safety, later using their wing claws to ascend back into the trees. These claws are lost as the bird matures. The hoatzin is a sexually dimorphic species with males and females differing in plumage and size. They are social birds, often nesting in colonies and exhibiting cooperative breeding behaviors. The hoatzin's evolutionary history is somewhat debated, with its exact placement within the avian tree of life being a subject of ongoing research.