The chicks of the hoatzin (Opisthocomus hoazin) are notable for possessing a pair of claws on each hand which enable them to employ a unique method of escaping predators; they drop into the water, swim to safety, and use the claws to clamber back up onto dry land. As you can see from the video, this also means that accidentally plopping into the water is no big deal (provided the chick climbs out before an aquatic predator snacks on it, natch). The claws disappear as the young birds approach adulthood.
[Image: The prominently-clawed hand of a hoatzin embryo (top) compared with the hand of an adult. Source.]
So why do hoatzins start out with wing claws even though most other modern birds lack them? Presumably, the instructions for making wing claws still lie dormant within any given clawless bird’s genome, a holdover from their non-avian dinosaur ancestors. At some point in the hoatzin’s evolutionary history, a mutation may have caused these genes to be expressed once more. Evolutionary throwbacks like this one are referred to as atavisms and have been observed in a variety of organisms, including humans. (Another avian example: chicken teeth!)
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