This story was originally published in February 2018.
Free time is dangerous for me. I ponder imponderable questions, like “Do birds sleep?” Well, yes, but not the same way we do, and each species has its own way of sleeping.
Sleeping is dangerous. Slumber too deeply, and you may become somebody’s unwary lunch. Most small birds grab a few winks as needed, but they are hard-wired to wake instantly if disturbed by a neighbor, strange sound, or approaching threat. Awake, gone.
Some birds can sleep with one eye open, resting half of their brain while the other half remains alert. It’s called unihemispheric slow-wave sleep. The ability is not limited to birds. Certain whales and porpoises have demonstrated the knack, too. It’s most common in species that sleep in open fields or water, like ducks and seabirds.
Some long-distance migrants are capable of sleeping on the wing, since they often need to stay aloft for days or weeks at a time. But are they really sleeping up there? The only way to know is to wire them up and measure brainwave activity. Obviously, that is impractical for tiny birds.
Frigatebirds are large birds of the tropical oceans. They resemble prehistoric pterodactyls, with short legs and very long, pointed wings that allow them to stay aloft for days. These birds are large enough to be fitted with brainwave monitoring devices. Though they can rest briefly on water, they are not comfortable there. With such long wings, frigatebirds face difficulties taking off from a flat surface, and their tiny feet give them no boost.
Studies show that frigatebirds do not sleep while afloat. Instead, they snooze aloft while circling on rising air currents. They keep one eye open to make sure they don’t bump into anything, but brainwave activity in one half of the brain slows way down.
Mallards are notorious for their ability to sleep with one eye open. While songbirds can hide in the bushes, ducks tend to congregate in open areas where extra vigilance is crucial. They often roost in flocks, making it more difficult for predators to sneak up. In fact, in large groups, the birds in the center of the circle apparently sleep more soundly, while birds at the edge of the flock show more restlessness.
Most birds enjoy periods of deep sleep similar to humans. This rapid-eye-movement (REM) sleep tends to be of much briefer duration in birds. For many birds, the amount of sleep they get corresponds to the length of darkness. It’s dangerous to move around at night, and sleeping is a good way to conserve energy.
Migrating songbirds face a particular challenge. Most migrate at night when it’s safer and cooler. By day, they must refuel, which leaves them with little time to sleep. One study of Swainson’s thrushes – a bird that nests in Maine – demonstrated that they solve their problem by taking catnaps throughout the day. They can spend up to 15 percent of the day taking one-minute snoozes.
Breeders on the arctic tundra, such as many sandpiper species, face the challenge of round-the-clock daylight. Male pectoral sandpipers have no role in raising the family, and they spend the courting season trying to mate with as many females as possible. Scientists were amazed to discover that the males have adapted to severe sleep deprivation during the three weeks of courtship. They were even more astounded to find that the males who slept the least produced the most offspring, despite the severe physical toll it took on them. Indeed, pectoral sandpipers can resist the urge to sleep longer than any other animal ever measured.
So why don’t sleeping birds fall off their perches? Their legs are constructed so that the toes automatically close around the branch when they bend their legs into a squatting, sleeping position. The toes mechanically open as the bird stands to take off.
So why do some birds sleep on one leg? All the muscular motion of an active bird generates heat, but a sedentary bird at roost needs to conserve warmth. Strategies include pulling one leg up into downy breast feathers to reduce heat loss. Another strategy is to lay their heads on their backs, burying their beaks into their feathers while sleeping. Not only does this allow them to stay warmer, but they can actually breathe air warmed by their own body heat. Waterfowl and wading birds often employ this tactic.
Well, that’s this week’s imponderable question. Next week: What is the airspeed velocity of an unladen African swallow?
