Wouldn’t it be great to dose 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 sleeping birds can keep one eye open and one half of their brain awake, a phenomenon called unihemispheric sleep.
Just like us, birds exhibit two types of sleep, slow-wave sleep (SWS) and rapid eye-movement (REM) sleep. SWS sleep can occur in one or both brain hemispheres at a time, but REM sleep only occurs in both hemispheres together. Scientists use an electroencephalogram (EEG) to measure the electrical activity of the brain and detect the different stages of sleep. Electrodes are attached to the bird’s head and connected by wires to a computer which records the brain’s electrical activity.
Take a closer look at the resting mallard ducks the next time you are at the local park. You will notice that some of them have one eye open and one closed. They are half asleep. Having the ability to sleep with one eye open might be a good adaptation for sleeping without getting eaten.
Niels Rattenborg, now at the Max Planck Institute for Ornithology, tested this idea using Mallards. The most risky place for a duck is probably at the edge of the sleeping flock. He lined up mallard ducks each attached to an EEG in a clear plastic cage with a video camera watching. He repeated this for four nights allowing each duck to be on the end of the row.
Ducks at the edge of the flock were 150% more likely to sleep with one eye open and usually that open eye faced outward from the group. They would stand up and turn around occasionally to shift sleep from one side to the other while maintaining a wide open eye to the outside.
Rattenborg didn’t stop there. He displayed an expanding video image of a predator attacking the flock to the half sleeping ducks. They immediately initiated a flight response to get away.
Many songbirds migrate thousands of miles between breeding and winter grounds each spring and fall. Incredibly, they do it all at night. When they land each morning they have to forage to refuel for the next night’s journey.
Rattenborg and his colleagues wondered how these migratory birds deal 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 migrating birds spent about two-thirds less time sleeping and fell into REM sleep much more quickly. They also performed cognitive tests in which they had to peck a key in order to get a seed. Sparrows sleep deprived during the nonmigratory period suffered cognitively, but they displayed an impressive ability to maintain cognitive functions when sleep deprived during the migratory state.
How do those birds rest for the next night flight? Thomas Fuchs of Bowling Green State University in Ohio kept Swainson’s thrushes, a forest songbird, 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 the shut-eye, they 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 can recuperate while only marginally increasing the risk of being eaten. Quick rests during the day appear to give the birds plenty of time to forage and replenish fat stores for the next night flight.
Scientists speculate that some birds might be able to catch up on sleep while in the air –unihemispheric sleep may be compatible with flight. 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 bird brain activity while they are in flight.
Aquatic mammals, like 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 dosing off?