Despite being able to sleep on the wing, when compared with sleep on land flying frigatebirds sleep very little, in shorter bouts, and less deeply. The ability to sustain cognitive performance on limited sleep challenges the dominant view that large daily amounts of sleep are required to maintain adaptive performance.
Certain birds like songbirds and migratory birds do not get the adequate amount of sleep since they are airborne day and night. For example, a common assumption is that albatrosses must be able to sleep in flight, although no direct evidence has ever been obtained. Scientists assumed that some bird species are capable of sleeping while still in flight.
Now we know it. A study by an international team of researchers has now proven this to be true, showing that birds rest while hitching a ride on rising air currents. The study released in the journal Nature Communication, found that frigatebirds sleep for an average of 41 minutes per day, even while they are in flight. They take quick 12-second naps, usually using only half their brain but sometimes all of it.
Researchers fitted 14 great frigatebirds (Fregata minor) flying over the ocean for up to 10 days with miniature brain activity monitors and GPS to record position and altitude,and found that they are capable of sleeping mid-flight
As Darwin observed during his voyage to the Galápagos Islands, frigatebirds are not known to rest on the water despite spending weeks to months flying over the ocean. Their long wings, poorly webbed feet and reduced feather waterproofing make taking off difficult following more than momentary contact with the water. To catch food, great frigatebirds rely on large predatory fish and cetaceans to drive prey, such as flying-fish and squid, to and above the surface.
Birds exhibit two forms of sleep: slow-wave sleep (SWS) and rapid eye movement sleep (REM sleep), just like mammals. SWS can alternate between both hemispheres of the brain but REM sleep occurs simultaneously in both. In SWS, the eye that is connected to the active hemisphere remains open.
This allows the bird to sleep but still remain aware of its environment. If it isn’t necessary for the bird to watch its environment, SWS that involves both hemispheres takes control. REM sleep, which causes reduction in muscle tone, isn’t likely when the bird is in flight, the study found.
Great frigatebird with a head-mounted data logger for recording the electroencephalogram from both cerebral hemispheres and a back-mounted GPS logger recorded position and altitude. By Bryson Voirin