Frigate birds can sleep during flight, study finds

Scientists have found birds are capable of sleeping in flight, according to a new study.

Published in the journal Nature Communications, the new study  looked at brain activity of frigatebirds that indicated the birds sleep in flight with either one side of their brain at a time or both side at the same time. Despite this ability, the birds in the study slept less than an hour per day, a small percentage of the time they spend sleeping on land.

How frigatebirds are capable of functioning on such little sleep remains a mystery, the researchers said.

To figure out if and how birds sleep in flight, the study scientists set out to record variations  in brain activity and behavior that differentiate wakefulness from the two kinds of sleep discovered in birds: slow wave sleep (SWS) and rapid eye movement (REM) sleep. The study team created a small device to gauge electroencephalographic (EEG) variations in brain activity and head movements in flying birds.

How did they track this?

The scientists briefly attached the small “flight data recorder” to the head of nesting female frigatebirds. The birds then toted the recorder during non-stop foraging flights, which lasted as many as ten days and as far as 1800 miles. During this period, the recorder tracked the EEG activity of both brain hemispheres and motions of the head, while a GPS device on the birds’ back followed their position and altitude. When birds were back on land and had had time to recuperate, they were captured and the equipment was taken off.

“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,” study author Bryson Voirin, a post-doctoral researcher at the Max Planck Institute in Germany, said in a press release.

The scientists discovered the birds remained awake while actively looking for foraging opportunities. Late in the day, the ‘awake’ brain pattern switched to a SWS pattern for several minutes while the birds were soaring. Remarkably, SWS could take place in one hemisphere at a time or both hemispheres together. The existence of this sleep suggested single-hemisphere sleep is not necessary to preserve aerodynamic control. However, when contrasted to sleep on land, SWS was more frequently on one side of the brain in flight.

Along with engaging in both kinds of SWS in flight, on uncommon occasions, rounds of SWS were disrupted by short installments of REM sleep.

The team also found frigatebirds slept an average of only 42 minutes per day while flying. In comparison, the birds slept on land for more than twelve hours per day. Furthermore, instances of sleep were lengthier and more intense on land. All together, this indicated frigatebirds are in fact sleep deprived in flight, the team said.

The researchers said it was unclear how the birds were able to function on such little sleep, and they said their future research would focus on this mystery.

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Image credit: University of Alabama

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