Migratory birds fly huge distances without touching the ground, sometimes staying aloft for weeks at a time.
Scientists had assumed that the birds were able to sleep mid-flight, but they had never conclusively observed them doing so. Now a new study involving frigatebirds in the Galapagos Islands has proved that they do.
Researchers look into frigatebird brain activity
A team of researchers from the Max Planck Institute for Ornithology tested the idea of “in-flight sleep” by fitting frigatebirds with small devices that measure brain activity.
Frigatebirds forage for weeks at a time over the ocean, looking out for flying fish or squid that have been forced to the surface by other predators like whales or fish. The GPS-enabled devices fitted to the birds are similar to “black box” flight recorders found on planes.
The devices recorded data as the birds made flights for around 10 days over distances of about 1,800 miles. Electroencephalographic information was collected from both hemispheres of the bird’s brain, and the devices also tracked the movement of their heads.
Data shows both brain hemispheres rarely shut down at once
Scientists collected the devices when the birds returned to their nests on the island, and processed the data. It showed that birds did indeed sleep in mid-air.
In daylight hours they remained alert and ready to forage, before resting during the night as they switched to short-wave sleep (SWS) for a few minutes at a time.
Scientists were surprised to find that the SWS activity could occur in both hemispheres of the brain at the same time. However the majority of the time they would rest one side of the brain while the other stayed awake. This is known as unihemispheric sleep, and is also practiced by dolphins among other species.
However the fact that both hemispheres shut down on some occasions shows that the birds did not need unihemispheric sleep in order to stay in flight. The reason for keeping one side of the brain alert is in fact to avoid collisions.
Collision-avoidance system kicks in
When birds were flying in a circle in a rising air current, the brain hemisphere linked to the eye on the inside of the turn would stay alert. This means that the birds were still keeping one eye working as they made the circle.
“The frigatebirds may be keeping an eye out for other birds to prevent collisions, much like ducks keep an eye out for predators,” said the study’s lead author, Niels Rattenborg, in a statement.
Interestingly these sleep episodes were not deep or long. Frigatebirds were found to sleep around 42 minutes per day in the air, compared to over 12 hours per day on land.
“Why they sleep so little in flight, even at night when they rarely forage, remains unclear,” said Rattenborg.
Rattenborg now wants to research how frigatebirds can get by on so little sleep and still forage successfully. “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,” Rattenborg said.
The full results of the study are published in the journal Nature Communications. The huge difference in the amount of sleep that frigatebirds get in the air compared to on land makes the subject incredibly interesting indeed.
Sleep has become something of a trendy subject, with productivity-obsessed individuals researching how to function on fewer and fewer hours of rest. Perhaps the frigatebirds hold the key.
Source: Nature Communications