Hoatzin

Hoatzins

The hoatzin ( pronounced watzin ) is a swamp living bird of South America that eats leaves of the arum-like plant moka-moka. The leaves accumulate in its crop & when full to the point of bursting, it flies its way to a roost where it sits, & now bacteria in its crop start fermenting the leaves and the muscular walls of the crop heave, until the leaves are reduced to a fatty smelly substance. Nearly two days later, the remains of this are excreted. Its excreta smells a bit like cowdung. So, the hoatzin has earned for itself a local name - the stink bird.
The hoatzin is unusual in that its young have two well-developed claws on each wing that they use to cling to branches & to rush back up to their nest in case of a scare from predators. They lose these claws when they become adults.
Probably, the extinct Archaeopteryx, a connecting link between reptiles & birds, which retained claws throughout its life, used them in the same way as do the hoatzin nestlings.