Field Guide
How birds sleep
Nocturnal fliers like whip-poor-wills and owls are just getting going when the sun sets. But what do birds that are active during the day do at night? Most find a secluded spot to catch some shut-eye; however, sleep in the avian world is different than our rest.
Birdy Catnaps
Instead of getting prolonged sleep, birds often take hundreds of short snoozes each day. In a variety of instances, birds are never really asleep—at least not in the way that people think. Half of a bird’s brain stays active while it is resting, all thanks to the phenomenon of unihemispheric slow-wave sleep. Remaining partially alert can help birds detect potential predators and adjust to changing environmental conditions.
Some birds even nap while flying. Magnificent frigatebirds, for example, sleep up to 12 hours per day when on land, but they are limited to less than an hour of rest per day when taking long cruises over the ocean, typically while they’re riding warm air currents. Niels Rattenborg and colleagues
report in Nature Communications that frigatebirds keep the eye that is connected to the awake and alert hemisphere of the brain facing the direction of flight. This gives a whole new meaning to thinking on the fly!
Built-in Blankets
Maintaining warmth is a challenge for sleeping birds. Feathers provide strong insulation, so birds tuck into themselves. As they snooze, ducks, geese and swans are often seen with their bills buried in their own feathers. Many species will also pull a leg up into their cozy little zones.
Birds rarely use nests as night roosts when they aren’t actively incubating eggs or babies. Only a few cavity nesters like nuthatches, wrens and chickadees take to nest boxes throughout the year. Acorn woodpeckers, bluebirds, pygmy nuthatches and tree swallows have all been documented communally roosting with small groups, each sharing the same cavities.
Perched for Shut-eye
The vast majority of songbirds are natural perchers. This holds true even while they rest. Chalk it up to a bird’s physique: To hold up its body weight, a bird will instinctively tighten its tendons and clamp its feet onto a branch.
Hummingbirds are the ultimate sleepers. Under extreme conditions, they’ve been found perched upside down, kind of like a bat. During cold spells, hummingbirds lower their metabolism and body temperature and enter an overnight state called torpor, which almost mimics hibernation in other animals.
While they’re dozing, it’s unclear if birds dream, although they can experience bouts of rapid eye movement. Research in zebra finches has shown that brain neurons associated with song can be activated during sleep, yet this could simply be a way of reinforcing song learning instead of proof of dreaming.