Birds sleep in mid-flight —

Great Frigatebird snatching prey item (Sooty Tern chick dropped by another frigatebird) from ocean surface

Great Frigatebird snatching prey item (Sooty Tern chick dropped by another frigatebird) from ocean surface

Many birds fly non-stop for days or longer, but do they sleep in flight and if so, how? It is commonly assumed that flying birds maintain environmental awareness and aerodynamic control by sleeping with only one eye closed and one cerebral hemisphere at a time. However, sleep has never been demonstrated in flying birds. Using electroencephalogram (EEG) recordings of great frigatebirds (Fregata minor) flying over the ocean for up to 10 days and spanning up to 3,000 km, a team of scientists have shown that they can sleep with either one hemisphere at a time or both simultaneously. It is also found that, while flying, they sleep for only 7.4% of the time spent sleeping on land. In addition to establishing that birds can sleep in flight, results challenge the view that they sustain prolonged flights by obtaining normal amounts of sleep on the wing.

Niels Rattenborg from Max Planck Institute for Ornithology in Seewiesen, Germany teamed up with Alexei Vyssotski from University of Zurich and Swiss Federal Institute of Technology who developed a small device to measure electroencephalographic changes in brain activity and head movements in flying birds.

In collaboration with the Galapagos National Park and Sebastian Cruz, an Ecuadorian seabird biologist, the team focused on great frigatebirds nesting on the Galapagos Island.

The study published in Nature Communications says, a diverse array of birds, including swifts, sandpipers, songbirds and seabirds, engage in non-stop flights lasting several days, weeks, or longer. Given the adverse effects of sleep loss experienced by most animals it is commonly assumed that birds fulfill their daily need for sleep on the wing. The recent discovery proves that some birds can perform adaptively for several weeks despite greatly reducing the time spent on sleeping.

On land, birds can switch from sleeping with both hemispheres simultaneously to sleeping with one hemisphere at a time. During unihemispheric slow wave sleep (USWS) birds keep the eye connected to the awake hemisphere open and directed toward potential threats. Flying birds rely on USWS to maintain environmental awareness and aerodynamic control of the wings, while obtaining the sleep needed to sustain attention during wakefulness.

Frigatebirds do not rest on water, as some birds do, 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. Consequently, they face ecological demands for wakefulness 24/7 while over the ocean.

By recording their brain activity while flying it has been demonstrated that they can sleep in flight with one hemisphere at a time or both together. Although frigatebirds engage in both types of sleep on the wing, sleep is more asymmetric in flight than when on land. They sleep mostly while circling in rising air currents and keep the eye connected to the awake hemisphere open facing the direction of flight, suggesting that they use unihemispheric sleep to watch where they are going. 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, suggesting that they face ecological demands for attention that usually cannot be met through sleeping unihemispherically.

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