Wouldn’t it be great to doze off at work and still get something done? If you had a bird brain, you just might be able to pull it off. Studies have shown that birds regularly keep half of their brain awake (and one eye open) while the other half of the brain sleeps, a phenomenon called unihemispheric sleep.
Just like us, birds exhibit two types of sleep: rapid eye-movement (REM) sleep and slow-wave sleep (SWS). Avian REM sleep occurs in both brain hemispheres at the same time, while SWS can occur in one or both brain hemispheres. Scientists use an electroencephalogram (EEG) to measure the electrical activity of a sleeping bird’s brain.
But you can see unihemispheric sleep for yourself, without any special equipment. Take a closer look the next time you see a groups of resting mallard ducks. You will notice that some of them have one eye open and the other closed. They are half asleep.
Having the ability to sleep with one eye open might be a good adaptation for sleeping without getting eaten, a hypothesis tested by Niels Rattenborg, now at the Max Planck Institute for Ornithology. The most risky place for a sleeping duck is at the edge of the sleeping flock. Rattenborg lined up mallard ducks, each attached to an EEG, in a clear plastic cage with a video camera filming. He repeated this test for four nights, allowing each duck to take a turn on the end of the row.
Edge ducks were 150 percent more likely to sleep with one eye open, and usually that open eye faced outward from the group. The ducks would stand up occasionally and turn around, shifting sleep from one side of the brain to the other while keeping the outside eye open.
Rattenborg didn’t stop there. He displayed a video image to the half-sleeping ducks of a predator attacking the flock. The “guard” ducks immediately sounded the alarm.
Many songbirds, as well as ducks, migrate long distances between breeding and wintering grounds each spring and fall. Incredibly, they do it all at night. When they land each morning, instead of catching up on rest, they have to forage to refuel for the next night’s journey.
Rattenborg and his colleagues wondered how these migratory birds function with such sleep deprivation. They captured white-crowned sparrows, small songbirds that breed in the far North and visit our backyard bird feeders during migration, and kept them under surveillance for a year in an aviary. During the migration season, they become restless in the aviary, hopping and flapping around.
Using EEGs to monitor their sleep patterns, the scientists found that, during times when they would normally be migrating, these birds spent about two-thirds less time sleeping and fell into REM sleep much more quickly than they did during the non-migratory period. The scientists also performed cognitive tests in which the birds had to peck a key in order to get a seed. During the non-migratory period, sleep-deprived sparrows suffered cognitively. But they displayed an impressive ability to maintain cognitive functions when sleep deprived during the migratory period.
So how do these birds get enough rest during migration? Thomas Fuchs of Bowling Green State University in Ohio kept Swainson’s thrushes, a forest songbird that nests in our region, in an aviary for an entire year to find out. Just like the sparrows, the thrushes became restless during the migration time and stayed awake the entire night.
To catch up on shut-eye, the thrushes took hundreds of daytime powernaps that lasted on average only nine seconds and rarely exceeded more than 30 seconds. The siestas ranged from full sleep to unihemispheric sleep to just a drowsy state. By alternating sleep types, they recuperated while only marginally increasing their risk of being eaten. Quick rests during the day appear to give the birds plenty of time to forage and replenish fat stores before the next night’s flight.
Scientists speculate that some birds might be able to catch up on sleep while in flight. Unihemispheric sleep may be compatible with flight, since one eye can be kept open for navigation. The reduction in muscle tone that usually accompanies REM sleep makes it unlikely that birds enter deep sleep in flight. With the recent advancement of miniature EEG recording devices, scientists will soon be able to measure brain activity while birds are in flight.
Aquatic mammals, including dolphins, seals, and manatees, can also sleep unihemispherically. Unlike birds, this adaptation keeps them from drowning. For example, dolphins swim mechanically in circles while asleep. Half of a fur seal’s brain stays awake, paddling a front flipper to keep a nostril above the water.
Studying animals that exhibit unihemispheric sleep could prove useful for understanding sleep disorders and sleep deprivation in humans. Do I sense you are dozing off?