Scientists have known for a while that many birds, including the buoyant frigate bird, can fly non-stop for weeks without having to rest on the ground.
In fact, songbirds, swifts, seabirds and sandpipers are also known to fly without stopping to rest for several days, weeks and even months as they travel across the world.
For humans, sleep deprivation can have adverse effects, so it has been assumed that these birds must fulfill their need for sleep on the wing.
As it turns out, these birds can sleep while flying mid-air thanks to some kind of special process in their brain, a new study revealed.
When a team of international researchers measured the brain activity of frigate birds, they found that the animals can sleep in flight either with both hemispheres at the same time or just one cerebral hemisphere.
Researchers say the findings offer insight on the concept and nature of sleep for birds compared to other animals.
Determining An Answer
In order to find out whether and how these birds sleep while flying, researchers recorded the brain activity changes that distinguish wakefulness from the two types of sleep in birds: the rapid eye movement (REM) and the slow wave sleep (SWS).
The team examined great frigate birds from the Galápagos Island and temporarily attached a small flight data recorder to the head of female frigate birds.
These animals then carried the flight data recorder as they traveled through non-stop foraging flights that lasted up to 10 days and covered a distance of 3,000 kilometers (1,864 miles).
As soon as the birds went back on land and had time to recover, researchers re-caught them and the flight data recorders were removed.
The study's first co-author Bryson Voirin said the frigate birds were remarkably calm and had even slept when he approached to catch them again for the second time.
In the end, data from the long flight revealed that frigate birds sleep in expected and unexpected ways. The birds remained awake during the day to actively search for hunting opportunities.
At sunset, the pattern switched to an SWS pattern for several minutes while the frigate birds were flying. The SWS could even occur in one hemisphere at a time or both hemispheres at the same time, researchers found.
This meant that sleeping with one hemisphere is not needed to maintain aerodynamic control, but that the birds were watching where they were going.
Niels Rattenborg, one of the researchers of the new study and an expert from Max Planck Institute for Ornithology, says the birds may be keeping an eye out for predators and other birds to avoid collisions.
Birds sleeping in flight might face the danger of colliding with other objects or falling from the sky. But researchers demonstrated that this does not necessarily happen.
Rattenborg found that when mallard ducks sleep at the edge of a group, the birds keep one brain hemisphere active and the corresponding eye open.
This open eye is directed away from other birds and toward a possible threat. Based on this data, scientists assume that birds depend on this autopilot process to navigate and keep aerodynamic control.
Still, it may also mean that the birds have evolved in a way that they can cheat sleep. Researchers discovered that male pectoral sandpipers that compete for mates can perform adaptively for weeks even with little sleep.
This raised the likelihood that the birds simply forego sleeping when up in the air, they said.
Meanwhile, although the birds can engage in all types of sleep while flying, they only sleep less than one hour every day — a small fraction of the time they spend sleeping on land, researchers say.
The reason as to why this happens still remains a mystery. Rattenborg and colleagues plan to investigate how frigate birds are able to sustain adaptive performance despite having little sleep.
The findings of the new study are published in the journal Nature Communications.