...About Sleeping Birds?

by Carl Hertzman

Recent studies indicate that a number of species of birds, particularly waterfowl, sleep with one eye open (called uni-hemispheric sleep) allowing one hemisphere of the brain to sleep while the other stays awake. Presumably the birds switch eyes during the night. The open eye focuses on the direction from which danger is expected. The bird can awaken and react instantly. In birds the optic tracts cross over to the opposite side of the brain. Thus they differ from humans where each eye is represented by both sides of the brain.

The length of time birds sleep depends on the length of night. They tend to retire up to an hour before darkness and wake up before dawn, even singing while it is still dark. This is presumably because they are hungry early in the morning. In northern winters, birds such as chickadees may sleep 14 to 15 hours; with their high metabolic rate this gives them little time to stoke up for the following night, since having enough fat reserves is critical to surviving a cold night. Therefore, if you are feeding birds, getting food out early in the morning and late in the afternoon is important. Chickadees can lose ten percent of their body weight on a cold night. Some birds, such as red-polls, store seeds in their cheek pouches and swallow them during the night to maintain their metabolism.

During cold nights, birds will seek shelter from wind: in evergreens, under the eaves of buildings, anywhere they can get away from cold wind. (When my wife, Mary, gets up early and raises her window shade, birds including a cardinal will fly out of the evergreen bushes along the house.) Grouse will dive into a snow bank, since snow is a good insulator. In the arctic tundra and plains, birds will find depressions or huddle behind small brush to get out of the wind. Woodpeckers, and some other species, go into cavities; woodpeckers tend to retire early and get up late and may peer out of the hole for awhile before flying off. Some species, such as bluebirds, will pile into a cavity such as a nest box, sometimes suffocating the individual on the bottom. Birds will also fly into a barn, particularly if it is warm with livestock.

During summer, birds sleep less, since night is shorter. In the arctic, where there are 24 hours of sunlight, birds still rest for a few hours around mid-night. Particularly in the tropics or in deserts, if the temperature becomes very high birds become inactive during midday to prevent overheating. (There is not much bird activity even in Cincinnati on a hot summer day!)