For birds that make long-haul flights, scientists have assumed but never conclusively observed that they must take naps during their journeys. Guess what? They do, according to a new study of frigatebirds from the Galapagos Islands.
Researchers from the Max Planck Institute for Ornithology put the "in-flight sleep" assumption to the test, outfitting frigatebirds with tiny, specially designed devices that measure their brain activity.
Frigatebirds are the perfect test subjects to wear the gear. The seabirds make long foraging flights of weeks at a time over ocean water, looking for flying fish and squid that have been harassed out of the water by animals such as whales or predatory fish.
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The GPS-enabled devices, bird versions of "black box" flight recorders on planes, gathered data on the frigatebirds as they flew journeys lasting up to 10 days and about 1,800 miles (2,896 kilometers). The scientists collected electroencephalographic information from both hemispheres of the bird's brain and also tracked head movements.
Once the birds returned themselves and their data recorders to their Galapagos nests and the numbers could be crunched, the scientists found that the birds did indeed sleep mid-flight. During the day, they stayed on the job -- awake and ready to forage as they flew. But, when night fell, they switched to a type of rest known as short-wave sleep (SWS) that could last up to several minutes at a time while they were flying.
The SWS brain activity, the team found, to their surprise, could occur in both hemispheres of the bird's brain at once. But, far more often the seabirds effectively switched off half of the brain while the other hemisphere remained alert, in a process called unihemispheric sleep (also practiced by other species such as dolphins).
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The fact that the birds did, albeit infrequently, sleep with both hemispheres shut down told the scientists that unihemispheric sleep was not required for the animals to remain in control of their flight, aerodynamically speaking.
Why keep half a brain alert, then? Collision avoidance.
The team observed that when the birds made a circle off a rising air current, the brain hemisphere associated with the eye closest to the direction of the turn stayed awake, meaning the birds kept watch, with one eye, while they performed the maneuver.
"The frigatebirds may be keeping an eye out for other birds to prevent collisions, much like ducks keep an eye out for predators," said the study's lead author, Niels Rattenborg, in a statement.
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The sleep episodes, however, were neither long nor deep. The researchers found the birds only slept about 42 minutes per day (on land, frigatebirds can sleep more than 12 hours per day), opting for sleep-deprived journeys.
"Why they sleep so little in flight, even at night when they rarely forage, remains unclear," said Rattenborg.
Longer term, the researcher hopes to learn more about how frigatebirds get away with it, how they successfully perform their foraging duties on so little sleep. "Why we, and many other animals, suffer dramatically from sleep loss whereas some birds are able to perform adaptively on far less sleep remains a mystery," Rattenborg said.
Findings on the birds' in-flight snoozing have Just been published in the journal Nature Communications.
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