What is the only bird that hibernates?

The common poorwill is the only bird known to hibernate. This bird is found in the southeastern portions of Canada and western United States.

The common poorwill heads toward part of the southwest and Mexico during the winter, so its hibernation, which is more like long periods of torpor, is unlike that of most mammals. It has been found hibernating in the California desert during cold spells, where it goes without food and lowers its body temperature close to that of its surroundings in order to survive. This is a unique adaptation only found in this particular species of bird.