This is the first time that birds have been observed sleeping in mid-flight.
For years, scientists have suspected that birds can sleep in mid-flight, as many species of birds are known to fly non-stop for days or even weeks. Some researchers have hypothesized that birds instead forgo sleep when flying for extended periods of time, arguing that sleep deprivation barely affects certain species. Due to the lack of studies monitoring the sleep patterns of flying birds, these hypotheses had previously been left unconfirmed. Now, however, according to a new study from the Max Planck Institute for Ornithology, researchers have finally found evidence that birds do indeed sleep while flying.
Headed by neurophysiologist Niels Rattenborg, the international team of researchers that authored the study spent time in the Galápagos Islands monitoring the brain activity of great frigatebirds (Fregata minor). The great frigatebird is a species of large seabird that can spend weeks flying non-stop over the ocean in search of food.
To record brain activity, the team attached a small device to the heads of frigatebirds while they were still on land. The device used electroencephalography (EEG) to identify if and when the birds were asleep while they flew over the ocean. After about 10 days of non-stop flight, the birds returned to land, and the researchers recollected the devices to observe the results.
The team predicted that the flying frigatebirds would exhibit unihemispheric slow wave sleep (USWS), a phenomenon in which animals sleep with only one hemisphere of the brain at a time, allowing them to keep one eye open to watch out for potential threats. Birds like the mallard duck (Anas platyrhynchos) use USWS while on land to remain aware of predators. Dolphins have also been observed exhibiting USWS, allowing them to sleep while they are still swimming.
As predicted, the frigatebirds were found to use USWS while flying, leaving one eye open as they circled over the ocean. "The frigatebirds may be keeping an eye out for other birds to prevent collisions much like ducks keep an eye out for predators," Rattenborg explained. Surprisingly, the frigatebirds were also found to exhibit bihemispheric sleep, in which both hemispheres of the brain are asleep at the same time. This means that frigatebirds are able to fly with both of their eyes closed. The monitored birds even experienced brief bouts of rapid eye movement (REM) sleep, although they lasted only a few seconds. During REM sleep, muscle tone is reduced, causing birds' heads to droop. Despite this muscle tone reduction, REM sleep was not found to affect the birds' flight patterns.
Although the frigatebirds did sleep for brief periods of time in mid-flight, they spent a majority of the flight awake. On land, frigatebirds can sleep for over 12 hours in a single day. While flying, however, they spent less than 3% of their time asleep, sleeping about 42 minutes per day on average. Mid-flight sleeping also occurred almost exclusively at night even though frigatebirds on land can sleep during the daytime.
Rattenborg and his team were excited by the results of the study but puzzled by the frigatebird's ability to function on so little sleep. “Why they sleep so little in flight, even at night when they rarely forage, remains unclear,” Rattenborg admitted. “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.”