Frigatebirds, an aquatic bird with a six-foot wingspan, can spend weeks at a time up in constant flight, up in the clouds. How is that even possible? As Trace Dominguez explains in today's DNews report, the birds sleep with one eye open. Literally.
Earlier this year, scientists from the Max Planck Institute for Ornithology in Germany fitted several frigatebirds with small EEG monitoring devices that measure brain waves. Then they cut the birds loose and recording data over 10 days while their feathered test subjects flew, nonstop, more than 3,000 kilometers.
The results were pretty amazing. It appears that frigatebirds sleep in the air by resting their brain for short durations -- one hemisphere at a time. In other words, they sleep with one eye open, while still flying. Data suggest the birds typically circle upward on air currents in the direction of their open eye, which is the one connected to the awake hemisphere. The Planck Institute researchers think they do this to watch where they're going and so they don't run into other birds.
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Actually, frigatebirds are not the only animal that sleeps half a brain at a time. It's a documented but unusual rest pattern called unihemispheric slow-wave sleep, and dolphins do it, too. A dolphin will shut down one hemisphere of its brain, leaving the other half of the brain to monitor breathing functions and keeping that eye open to monitor its environment. The left eye will be closed when the right half of the brain sleeps, and vice versa.
We humans can actually do this, as well -- kind of. Sleep researchers have documented unihemispheric slow-wave sleep in people, though it's not a full hemispheric separation. It's more of an asymmetry and it's associated with the first night effect, in which people have trouble getting a good night's sleep in a new location. One hemisphere of the brain remains more active and vigilant, monitoring the environment for unfamiliar sounds in the new surroundings.
So that old expression about sleeping with one eye open? It's actually pretty accurate. Double Secret Bonus Fact: Because of their tendency to steal food from other seabirds, frigatebirds are classified as kleptoparasites.
-- Glenn McDonald
NPR: Nonstop Flight: How The Frigatebird Can Soar For Weeks Without Stopping
Science Alert: Scientists have just seen birds sleep during flight for the first time ever
Nature: Evidence that birds sleep in mid-flight