Attachable brain recorders reveal that birds can grab quick naps during flight, sometimes using only half of their brain.
Many bird species stay airborne for several days, weeks, or even months at a time as they traverse the globe. Frigatebirds, for their part, can spend weeks flying over the ocean in search of fish and squid near the surface. It’s difficult to imagine how any animal, much less one that typically sleeps for more than 12 hours per day on land, could go without any sleep for weeks on end. So a team of researchers set out to catch these birds in the act of napping during flight.
Focusing on frigatebirds nesting on the Galápagos Island, researchers attached temporary flight data recorders to the heads of 15 females. The birds carried the recorders, which logged the electrical activity of both of their brain hemispheres, during their non-stop flights that lasted up to 10 days and spanned as far as 3,000 kilometers (1,864 miles).
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On the birds’ return, the researchers retrieved the recorders. Bryson Voirin of the Max Planck Institute for Ornithology in Germany, co-author of the study published in Nature Communications, recalls in a press release, “Like many other animals in the Galápagos Islands, the frigatebirds were remarkably calm and would even sleep as I approached to catch them for the second time."
The recorder data revealed that the birds stayed awake during the daytime when they were actively searching for food. As night fell, their brains showed distinct signs of slow-wave sleep, which is a deep sleep, but not quite as deep as REM sleep. Incredibly, sometimes this sleep pattern only occurred in one brain hemisphere at a time — the birds were engaging in a kind of half-sleep.
Staying alert with one half of the brain while resting the other may be necessary in certain mid-flight situations. For example, when the birds circled on rising air currents, the hemisphere controlling the eye facing the direction of the turn usually stayed awake while the other slept, suggesting that the birds needed to keep an eye on where they were going to prevent collisions.
On rare occasions, the birds even slipped into REM sleep, but these episodes only lasted a few seconds at a time. Study lead author Niels Rattenborg notes that in contrast to mammals, birds are able to maintain some level of muscle control during REM sleep. Though their heads dropped momentarily during this phase, the frigatebirds were able to maintain normal flight patterns.
The study shows clear evidence that birds can indeed sleep as they soar through the sky. However, their 42-minute sleeps on flight days pale in comparison to their usual half-day slumbers on land, suggesting that birds on the wing are sleep deprived. "Why they sleep so little in flight, even at night when they rarely forage, remains unclear," says Rattenborg.
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