Hoatzin: The strange 'stinkbird' born with clawed wings that appears to be an evolutionary 'orphan'

a hoatzin bird leaping in the air with blue sky background — Hoatzins are considered evolutionary orphans as it's unclear where they sit within the avian family tree. (Image credit: Patrick Gallet / 500px/Getty Images)

Name: Hoatzin (Opisthocomus hoazin)

Where it lives: Amazon and Orinoco river basins in South America

Why it's awesome: The hoatzin is often regarded as one of the world's strangest birds. Hatchlings are born with clawed wings — a rare, prehistoric trait — and adults give off a strong, unpleasant smell due to their cow-like, fermenting digestive system, earning them the nickname "stinkbird."

Unlike most birds, the hoatzin has a foregut fermentation system, similar to the one found in cows. It primarily feeds on leaves, which it stores and ferments in a large, chambered crop — a temporary food storage pouch located in the esophagus. Then, the food is passed to the stomach for fermentation, where bacteria break down the tough plant material, releasing gases via burps that produce the bird's distinctive manure-like odor.

a gropu of hoatzin sitting together on tree branches in the rainforest — Hoatzin smell because of their cow-like fermenting digestive system. (Image credit: Nature Picture Library/Alamy)

This digestive process is highly efficient for breaking down the high levels of cellulose found in leaves. But, it makes the hoatzin both smelly and clumsy, as its enlarged gut makes it difficult to fly. However, the hoatzin's unusual smell acts as a natural defence mechanism, as predators tend to avoid the bird because they think it is rotten or poisonous.

Researchers are uncertain about the evolution of hoatzins. In 2015, a genetic study in the journal Nature suggested that the hoatzin is the last surviving member of a bird line that branched off the evolutionary tree in its own direction 64 million years ago, shortly after the extinction event that wiped out the non-avian dinosaurs.