Asleep at the wing? How frigatebirds can nap while flying

For the first time, researchers observed birds sleeping while in flight.

Some bird species can fly for weeks on end without ever touching ground. Scientists have long suspected that they compensate by taking in-flight naps, but none have actually observed it. Until now, that is.

“Most people assumed that birds sleep unihemispherically in flight,” says lead author Niels Rattenborg in an email to The Christian Science Monitor. “However, the frigatebirds also sometimes slept with both halves of the brain at the same time.”

Researchers from the Max Planck Institute for Ornithology tracked the brain activity of 15 frigatebirds over 10 days and 3,000 miles. According to their research, published Wednesday in the journal Nature Communications, the tropical seabird is capable of varying degrees of sleep while flying.

In most instances, the birds rested just one hemisphere of their brain, thus maintaining limited alertness. But for very short durations, they also could also fall into deep REM sleep with no effect on flight performance. Dolphins, who also sleep unihemispherically, are the only other animal known to sleep while moving.

“A surprising finding was that a majority of sleep was detected when animals were moving up by a rising airflow in a thermal, and not during straight gliding downwards,” says co-author Alexei Vyssotski in an email to the Monitor. “It is easy to explain by safety reasons: when a sleeping animal is moving up, there is no danger of a collision with the water.”

But perhaps the most surprising revelation was that, despite being able to, frigatebirds slept “remarkably little” on the wing.

It’s hard to grasp from a mammal-centric viewpoint, but the concept of sleep actually varies widely across different species. Humans, for example, rapidly deteriorate in the absence of sufficient rest. But many bird species, such as the frigatebird, function on hardly any sleep at all.

“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. “A surprising conclusion has been derived, that birds are lacking so-called ‘sleep-deprivation syndrome.’ ”

One theory is that avian bodies have evolved to undergo certain restorative processes – the ones that usually occur during sleep – while conscious. Birds may prefer to take a nap when possible, but it’s not required. To understand more, Rattenborg and Vyssotski will continue to study the connections between avian physiology and sleep.

“Determining why some animals can perform adaptively on little sleep, while others rapidly become compromised, may lead to new insight into the functions of sleep and the consequences of its loss,” Rattenborg says.