Half-awake or fully awake in flight?
Advertisement
Once the birds returned themselves and their data recorders to their Galapagos nests and the numbers could be crunched, the scientists found that the birds did indeed sleep mid-flight. This was seen in mallard ducks on land.
That’s because, without knowing what goes on in the sky, scientists weren’t sure whether birds were staying awake through their journeys, or just resting one hemisphere of their brain at a time – something that ducks have already been observed doing on land to avoid predators. “Unihemispheric sleep allows an animal to get some rest, while also allowing it to maintain awareness of its surroundings”, according to a recent Gizmodo News report.
Still, it may also mean that the birds have evolved in a way that they can cheat sleep. “However, sleep has never been demonstrated in flying birds”, the researchers wrote.
On rare occasions, the birds even slipped into REM sleep, but these episodes only lasted a few seconds at a time. The study team created a small device to gauge electroencephalographic (EEG) variations in brain activity and head movements in flying birds.
Headed by neurophysiologist Niels Rattenborg, the global team of researchers that authored the study spent time in the Galápagos Islands monitoring the brain activity of great frigatebirds (Fregata minor). Frigatebirds are large seabirds that spend weeks flying non-stop over the ocean in search of flying fish and squid driven to the surface by predatory fish and cetaceans.
The devices recorded data as the birds made flights for around 10 days over distances of about 1,800 miles.
Due to long-haul flights that can often last several days, weeks, or even longer, some sea birds have always been suspected of being able to fall fully asleep while “on the wing”, though it was not proven. Some researchers have hypothesized that birds instead forgo sleep when flying for extended periods of time, arguing that sleep deprivation barely affects certain species.
The flight data recorder revealed that frigatebirds sleep in both expected and unexpected ways during flight.
“Previous laboratory studies have shown that pigeons can stay awake for weeks without visible health problems, contrary to rats which will die under identical conditions”, says Dr. Vyssotski. Remarkably, SWS could take place in one hemisphere at a time or both hemispheres together. The presence of such bihemispheric sleep indicates that unihemispheric sleep is not required to maintain aerodynamic control. However, on land, the birds sleep normally. Mid-flight sleeping also occurred nearly exclusively at night even though frigatebirds on land can sleep during the daytime. But, far more often the seabirds effectively switched off half of the brain while the other hemisphere remained alert, in a process called unihemispheric sleep (also practiced by other species such as dolphins).
Advertisement
These short bursts of sleep isn’t uncommon among species of birds, but that isn’t to say that they are well rested with the average 42 minutes of sleep per day. That might seem like an extremely risky thing to do while flying, but it’s important to realize that REM sleep in birds isn’t like it is in mammals. In addition, although a reduction in muscle tone can cause the head to drop during avian REM sleep, birds are able to stand (even on one leg) during this state.
