Caught napping: First evidence that birds sleep during flight

On average the frigatebirds monitored in the study were only sleeping for 42 minutes per day(Credit: B. Voirin)

We already know that some birds can fly for weeks or even months at a time without landing, but this remarkable ability has raised a few questions about how, if at all, these creatures find the time to sleep. In the first study of its kind, scientists have monitored the brain activity of seabirds in flight and discovered that they regularly squeeze in some shut-eye while out searching for food, though how they perform on such little rest remains a little unclear.

If there was ever a bird well-suited to sleeping on the job, it might be the frigatebird, a large seabird that scans the ocean surface for flying fish and squid. Recent research has shown that these elite gliders can stay aloft for months by hitching rides on clouds and are among the longest-flying creatures in the seabird world. But even frigatebirds need their sleep, so scientists have been perplexed as to how they maintain performance without regularly coming down for rest.

When facing a dangerous situation on land, the mallard duck will only switch off half its brain when dozing off, literally sleeping with one eye open. The cerebral hemisphere left awake faces away from the other ducks and towards the potential threat so they can better prepare for an assault. Dolphins have also demonstrated this trick, known as unihemispheric slow-wave sleep. Scientists have suspected a similar technique might be at play with the frigatebirds.

But some birds, such as male pectoral sandpipers, can perform for weeks on very little sleep. So continuous flight in itself isn't necessarily evidence of the frigatebirds snoozing while in the air.

To find some conclusive answers, an international team of scientists hooked up frigatebirds nesting on the Galápagos Island to a device to monitor electroencephalographic (EEG) activity and head movement during flight. This recorder was carted along for 10-day flights across 3,000 km (1,864 mi) with a GPS module on the bird's back to monitor their position and altitude.

The data showed that during the day, the birds remained awake while searching for food, so business as usual. But when the sun went down and the birds soared, the awake EEG pattern changed to a slow-wave sleep (SWS) pattern, sometimes for minutes at a time. This SWS often occurred in one half of the brain, but interestingly, sometimes in both hemispheres at the same time, suggesting that unihemispheric sleep isn't critical to maintaining aerodynamic control.

Compared to how frigatebirds sleep on land, however, the SWS sleep mode was more frequent. By tracking the head movements of the birds, the researchers found that as they circle on rising air currents while sleeping in this way, it allowed them to keep one eye open in the direction they were turning.

"The frigatebirds may be keeping an eye out for other birds to prevent collisions much like ducks keep an eye out for predators," says Niels Rattenborg from the Max Planck Institute for Ornithology.

Every now and then during these SWS naps, the frigatebirds would slip into REM sleep. This only lasted a few seconds at a time, compared to the much longer REM sleep periods in mammals where muscle tone is lost. The birds did exhibit some loss of muscle tone with their heads dropping momentarily, but impressively, their flight was unaffected.

Perhaps the most surprising finding, however, was that on average the frigatebirds were only sleeping for 42 minutes per day, compared to the 12 hours a day they sleep, and much more deeply, on land. Clearly these birds do value their sleep, so with no Starbucks on hand how do they continue to function when so deprived of slumber?

This is what the scientists are now trying to figure out. Working out the answers and reconciling that with what we know about the importances of sleep in other animals could offer new insights into our understanding of sleep and sleep deprivation.

"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," says Rattenborg.

The team's study appears in the journal Nature Communications.

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