Half-asleep birds choose which half dozes.

Birds that are literally half-asleep--with one brain hemisphere alert and the other snoozing--control which side of the brain remains awake, according to a new study of sleeping ducks.

Earlier studies have documented half-brain sleep in a wide range of birds. The brain hemispheres take turns sinking into the sleep stage characterized by slow brain waves. The eye controlled by the sleeping hemisphere droops shut, while the wakeful hemisphere's eye stays open and vigilant. Birds also can sleep with both hemispheres resting at once.

To check whether birds can control half-brain sleeping, Niels C. Rattenborg and his colleagues from Indiana State University in Terre Haute filmed a row of mallards napping. Decades of studies of bird flocks led researchers to predict extra vigilance in the more vulnerable, end-of-the-row sleepers. Sure enough, the end birds tended to keep peeled the eye on the side away from their buddies. Mallards snuggled into the inner spots showed no preference for gaze direction.

Also, birds dozing at the end of the line resorted to single-hemisphere sleep, rather than total relaxation, more often than inner ducks did. Rotating 16 birds through the positions in a four-duck row, the researchers found outer birds half-asleep during some 32 percent of snoozing time versus about 12 percent for birds in internal spots.

"We believe this is the first evidence for an animal behaviourally controlling sleep and wakefulness simultaneously in different regions of the brain," the researchers say in the Feb. 4 NATURE.

The results provide the best evidence yet for a long-standing conjecture that single-hemisphere sleep evolved as creatures scanned for predators, Rattenborg says. The preference for opening an eye on the lookout side could be widespread, he predicts. He's seen it in a penguin pair napping side-by-side in the zoo and in a pet cockatiel perched by a mirror. The mirror-side eye closed as if the reflection were a pal, and the other eye stayed open.

Useful as half-sleeping might be, it's only been found in birds and such aquatic mammals as dolphins, whales, seals, and manatees. Presumably, keeping one side of the brain awake allows a sleeping animal to surface occasionally to avoid drowning, explains Rattenborg.

Nigel Ball, clinical director of the Sleep Disorders Center at the Virginia Mason Medical Center in Seattle, says the new study "gives us the possibility of looking outside the box." Studies of birds may offer unique insights into sleep because their lineage parted company from mammals' so long ago.

Jerome M. Siegel of the University of California, Los Angeles says he wonders if bird half-brain sleep "is just the tip of the iceberg." He speculates that a closer look at other species might turn up more examples.

Siegel, however, would be surprised to see the half-brain phenomenon during the rapid-eye-movement, or REM, phase of sleep. Birds and many mammals, including the primitive platypus, exhibit signs of REM sleep, which in humans has been associated with dreaming. So far, no one has found a creature half-dreaming.