For the Birds Radio Program: How do birds sleep?
(Recording of a American Crow)
One question which no listener has ever asked me, but which I’m sure has kept at least a few people awake nights wondering about is, “How do birds sleep?” I’ve watched injured birds long enough to know that they sleep extremely lightly. Part of the reason for this is that active birds tend to sleep more soundly that sedentary ones. But even the most active birds tend to sleep lightly enough to notice danger. Many times at night in the woods I have heard birds sing a soft song, perhaps aroused by some sound of possible danger, or maybe just talking in their sleep. Geese, which are louder than songbirds, are said to have alerted the ancient Romans of a night attack in the fourth century B.C., and even today geese are used to guard a distillery in Scotland because they sleep lightly enough to notice intruders.
Most birds use a living down pillow to rest their head–that is, they turn their head and rest it on their back or wing. Songbirds can perch on branches while in deep sleep–the flexor tendons of their feet shut tight when the leg is bent, locking the foot into position for the night. Many water birds sleep on shore on one foot. This is a handy adaptation for ducks and other birds with webbed feet, to prevent frostbite in winter. Ducks, loons, geese, and other swimmers can sleep like this on land or rocking in their natural waterbed. Many young birds and large flightless birds like the Ostrich and Rhea sleep with their head and neck extended along the ground. Woodpeckers, chickadees, bluebirds, nuthatches, and some other groups of birds sleep in cavities or birdhouses. A few birds will also use old nests for sleeping in, but most land birds sleep in trees or dense weeds. The few roosting birds I have found perching were tight against the trunk of trees, where they seemed least likely to present a noticeable silhouette for owls and other night hunters. In summer, when evenings are warm, birds sleep with smoothed feathers, but in winter they fluff out their down for warmth.
There is some circumstantial evidence that swifts and oceanic birds called Sooty Terns sleep on the wing, but some ornithologists doubt that this is really possible.
During sleep, the eyelids of a bird usually blink slowly. Two ornithologists studying Barred Owls concluded that they sleep when their eyelids remain fully closed, and merely doze when their eyelids are only partly closed. Herring Gulls apparently don’t keep their eyelids all the way shut for more that 60 seconds at a stretch, but there does appear to be a difference in the quality of their sleep depending on whether they tuck their bills under their shoulder feathers or face forward. Birds also seem to sleep more deeply when they are sitting than when they are standing. Birds living in the Arctic or Antarctic summer often spend the entire daylight period awake, which means that they don’t get much sleep all season.
Sleep habits are hard to study in birds because they awaken so easily when they are observed. And nobody has the foggiest idea whether birds actually dream. It’s hard to theorize about what bird dreams might be–their real lives are so fairy like that their fantasy lives are beyond my imagination.
(Recording of a American Crow)