Many birds have claws. These include adult flightless birds such as emus, cassowarys, osterichs and kiwis. Also, many other birds sometimes exhibit vestigal claws, including swifts, falcons and many others. In fact it is quite common, but most lose them in adulthood. The hoatzin appears to be one of the few, possibly only one, where the claws are actually functional.
The hoatzin is a prehistoric-looking bird related to the cuckoo that lives in small groups beside Amazonian lakes and swamps. The hoatzin is a medium-sized, striped, grey and white bird, and has a crest like a roadrunner, another cuckoo relative. Mating pairs are aided in rearing their oung by other group members.
Hoatzin young have moveable claws in the "elbows" of their wings. When confronted by a potential predator, juveniles that have not yet fledged (learned to fly) drop out of the nest and into the water.
When the coast is clear, they will swim back to the bushes or trees, clambering back up through the branches or trees to the nest by using the elbow claw. Eventually, the claw falls off as the young hoatzin matures.
Ornithologists believe this wing claw is a holdover from the most primitive bird form, Archaeopteryx, which was more reptile-like than what we know as a modern bird. Hoatzins are unique for another reason: they are the only bird with a ruminative-type digestive system, meaning it regurgitates plant material then chews it again to better digest the food.
