The Opisthocomidae family has only one living species—the Hoatzin (Opisthocomus hoazin). It lives in the swamps, rivers, and rainforests of the Amazon and Orinoco River basins in South America.

Hoatzins live in small family groups and are folivores, which means they eat mostly leaves. They also eat flowers, buds, and some fruit.

Because leaves are difficult to digest, the Hoatzin has a very unusual digestive system. It has a large crop, a pouch in its esophagus where food is stored. Inside the crop, helpful bacteria break down the leaves in a process called foregut fermentation. This digestive system is similar to the way cows, sheep, and other ruminant mammals digest food. The Hoatzin is the only bird known to use foregut fermentation.

The Hoatzin is about the size of a chicken, measuring 24–26 inches (61.0–66.0 cm) long. It has a bright blue featherless face, red eyes, a spiky crest, a long neck, and a long tail.

Young Hoatzins have two claws on each wing that help them climb through branches and back into trees if they fall into the water. As they grow, these claws become much smaller and are no longer useful.