Birds Asleep

by Mike Stiles

I would venture to say that most birders have not given much thought to how birds sleep. That was certainly the case for me, but in researching this article, it turns out that the habits of birds in the dark part of their lives is quite fascinating.

American Avocets

The vast majority of birds are diurnal and, unlike mammals with a sense of smell must rely on their vision, which, of all senses is the least effective in the absence of light. As night falls, birds must find a place to spend the night without being seen by predators, and must find protection from the cold night air. Many roost in trees or shrubs, some sleep on the ground or even under the ground, some sleep floating on water, and it is theorized that some even sleep while in the air. Some sleep singly and others roost in colonies that can number in the thousands.

If you’ve ever wondered how a bird can sleep on a perch without falling off, they have a tendon that will contract the toes around the branch when the knees are bent. As the bird crouches into sleep, the toes will grasp the perch with no muscular effort expended.

It is remarkable that tiny passerine birds can survive subfreezing nighttime temperatures, especially in the cold northern latitudes. One study of Winter Wrens (our local representative is now renamed Pacific Wren) found that they may travel up to a mile to gather with companions on cold nights. They may sleep in old nests of other birds, among dense foliage, in crevasses, or even in man-made structures. In one report, 31 wrens were found crammed into a nest box measuring six inches square. Some of the birds in the middle of these large masses have been found dead, presumably suffocated by the pressure of the outlying birds trying to find warmth.

Our local Pygmy Nuthatches will sleep in family groups, and the White-breasted Nuthatch, normally a solitary sleeper, will seek bedmates in chilly weather. Wrentits have been found on branches huddled together in a solid mass of bird, with their feathers intertwined.

Birds that nest on or near the ground will also sleep on the ground. Meadowlarks will sleep singly, scattered around the grassy field where they reside, and Savannah Sparrows will sleep in small, compact groups. Horned Larks will dig a depression in the soil just deep enough so that their backs are at soil level, and in soft soil will dig a new hole each night. As might be expected, sleeping on the ground is particularly dangerous, exposed to nocturnal mammals and reptiles, and the birds that do roost on the ground have been found to be more active and sleep less soundly than their tree roosting cousins.

Birds, and some marine mammals, can fall asleep in one side of the brain only. Called unihemisphere sleep, one hemisphere of the brain will be awake while the other half sleeps. Flocks of ducks have been observed with the individuals in the outer edges of the flock sleeping with one eye open. One half of the brain remains alert for predators. Swifts, who spend a majority of their life on the wing (and will eat, drink and even mate in flight), and many ocean going birds are thought to use unihemisphere sleep while flying.

Some sources think that it is only conjecture that birds will sleep while flying since it’s difficult to record the brain waves of a flying bird. Certain terns that cannot land on the water because their feathers will soon become waterlogged have been recorded staying in flight for many months. That is not positive proof that they sleep on the wing, but I suppose it’s either that, or they don’t sleep at all.

My source for much of this article is the book Birds Asleep by Alexander F. Skutch, illustrated by John Schmitt. John is an acquaintance of mine and gave me the book years ago. It’s worth a read, and I’m glad I finally picked it up.

Burrowing Owl on banner by Cleve Nash.