Sleep tight

Half of a bird's life is hidden from human view.

We have large gaps in our knowledge about bird behavior when they're sleeping and resting. And birds work hard to keep it that way. It's rare to encounter a sleeping bird, because most birds are careful to cover their tracks on the way to and from their well-hidden sleeping spots.

Many birds become increasingly nervous, even fearful, as dusk approaches, because nighttime is a dangerous time for them. When they sleep, they're extremely vulnerable to predators. Owls and other creatures that work the night shift are always on the lookout for a quick meal, and a sleeping bird is an easy target.

Sleeping quarters

That's why birds are careful to select sleeping sites that offer protection from predators -- and weather. Because they want these same protections for their nests, it's not surprising that many birds roost in habitat similar to their nesting sites.

Outdoor-nesting songbirds, such as cardinals, catbirds and robins, lock their feet onto tree branches or twigs in dense shrubbery and doze away the night. Cavity-nesting birds, such as woodpeckers, chickadees and nuthatches, snooze in tree holes. Some birds that nest on the ground sleep there, as well, including meadowlarks, white-throated sparrows, towhees and juncos.

Chimney swifts swirl down into tall chimneys or hollow trees, while many ducks float on water during the night, slowly paddling in place. Crows and other blackbirds gather in large groups, roosting together at the tops of trees. The northern bobwhite has one of the most fascinating sleeping styles in the bird kingdom. These small quail gather in a circle on the ground, heads out and tails in, ready to launch themselves into flight if a fox approaches.

Half asleep

We do know some fascinating things about birds and sleep.

Like marine mammals, birds seem to be able to maintain a "half sleep," which allows them to be at least partially alert to danger. Studies show that many bird species can sleep with one eye open and half of their brain awake. They can switch between brain hemispheres to give one half a rest, then turn both off to enter full sleep if they feel safe.

It's also known that the amount birds sleep varies with the seasons (they typically sleep longer in winter) and that nesting and migration seasons push them to the brink of exhaustion. During migration, most songbirds travel at night, which causes a major sleep deficit. They often compensate by taking lots of small "power naps" during the day, most only a minute or two in duration. A few birds in the swallow family may even sleep while coasting in the air.

Some birds "talk" in their sleep, making soft sounds. Others silently rehearse their species' song repertoire while sleeping. (This was revealed when scientists tracked the brain activity of several species.) Often, young birds will sleep bunched up on a branch with their siblings for the first few weeks after fledging, re-creating the shoulder-to-shoulder contact they experienced in the nest.

As it heads into a tangle of shrubs, a tree hole or leaf pile to spend the night, a bird has as its primary goal waking up to greet the morning. Birds improve their odds by carefully selecting their sleeping quarters and coming and going without attracting attention.

Sleep aids

We can help birds get a good night's sleep by providing a variety of places for them to roost in our back yards. Here are some suggestions:

Put up roost boxes (attached to trees or posts) for cavity nesters. Roost boxes are available at a wild bird store. If you'd like to build your own, check the Internet for building plans.

Plant evergreens for a natural year-round shelter. Arborvitaes are a good alternative for yards that lack space for a full-sized conifer.

Make brush piles from tree branches and twigs, the denser the better. They provide good shelter all year, even in winter.

Leave nest boxes out. Some birds will use them as nighttime roosts after the breeding season ends.

Val Cunningham, a St. Paul nature writer, bird surveyor and field trip leader, can be reached at valwrites@comcast.net.