Aristotle's second and more enduring hypothesis was that birds hibernate.

Aristotle’s second and even more enduring hypothesis was that birds hibernate. This isn’t so far-fetched. Some species do enter short, deep sleeps which lower their heart rates and metabolisms. And there’s at least one truly hibernating bird: the common poorwill sleeps out winters in the deserts of North America. But researchers were proposing much more outlandish forms of hibernation well into the 19th century. Barn swallows were said to remove their feathers and hibernate in holes, or sleep through the winter at the bottom of lakes and rivers.