There's some birds that use deep sleep as a survival tactic, but there’s a world of difference between a deep sleep and hibernation proper. During cold or wet weather, parent swifts find it hard to catch enough airborne insects, so their chicks back at the nest chill themselves, reducing their metabolism to go without food for 48 hours – enough to survive until the front passes.
But this is semi-torpor, not hibernation. Many normally hyperactive hummingbirds do something similar, entering a state of suspended animation. Their metabolism slows right down, their breath hardly detectable.
Again, however, this is an example of torpor. Just one bird is known to be a true hibernator: North America’s common poorwill. This beautifully camouflaged nocturnal bird is a relative of the nightjar found in Britain, and in winter often hibernates among rocks.
It can slash its oxygen intake by 90 per cent, while its body temperature plummets to 5°C, barely registering signs of life.
Main image common poorwil © Louis Agassiz Fuertes (artist, 1874-1927), Public domain, via Wikimedia Commons
