Several mammals, including rodents and bears, hibernate over the cold winters. But at least one bird does so as well.
Several animals go into a metabolic torpor in order to conserve energy and wait for food to become abundant again. Black bears and brown bears are probably the most famous hibernators, but prairie dogs, hedgehogs, and other small mammals also find places to wait out the winter cold.
(Or, in some cases, they wait out excessive heat . Some snails, frogs, ants, moths, and crabs “aestivate,” that is, they hibernate through summer.)
It was long believed that birds hibernated in winter. This belief was convincingly refuted when a stork showed up in Germany with an African arrow through its neck. But there is one exception: the common poorwill of North America. This bird will bury itself in a rock pile and drop its metabolism during winter. Heart rate, breathing, and body temperature all drop… and it can stay that way for weeks or even months. It is, effectively, the only hibernating bird.
Incidentally, the poorwill’s hibernation was only confirmed in the 1940s, but Lewis – of the Lewis and Clark expedition – had this to say on October 16, 1804:
This day took a small bird alive of the order of the [blank] or goat suckers. It appeared to be passing into the dormant state. On the morning of the 18th the murcury was at 30 a[bove] 0. The bird could scarcely move. I run my penknife into it’s body under the wing and completely distroyed it’s lungs and heart – yet it lived upwards of two hours. This fanominon I could not account for unless it proceeded from the want of circulation of the blo[o]d.Journals of the Lewis and Clark Expedition
And going back even further, the Hopi name for the poorwill is “hölchko” – the sleeping one.