Given the enormous distances that some birds travel while foraging for food, scientists have often wondered how these feathered voyagers find time to sleep when on their mammoth journeys. It has previously been hypothesized that some birds actually sleep mid-flight, yet keep one half of their brain awake at all times in order to monitor where they are going. For the first time, researchers have now observed the brain activity of frigatebirds in flight, confirming that they do indeed take power naps while soaring through the air.
Surprisingly, however, the team discovered that the birds occasionally shut off both hemispheres of their brains, indicating that it is not necessary to keep one half of the brain alert in order to maintain aerodynamic control. Perhaps even more unexpectedly, however, the researchers found that frigatebirds only sleep for a total of 42 minutes a night when on long journeys (compared with 12 hours on land), implying that they are actually sleep-deprived for most of this time, yet somehow still able to function effectively.
The team developed a small device to measure the electrical activity of the birds’ brains, which they attached to the heads of frigatebirds in the Galapagos Islands. Tracking the movements of the birds using GPS, the researchers monitored their movements as they spent up to 10 days at a time on foraging trips over the ocean, covering up to 3,000 kilometers (1,860 miles) before coming back to dry land.
Frigatebirds often fly in groups, so they have to sleep with one eye open to avoid crashing into each other. Jess Kraft/Shutterstock
While away, the birds remained constantly awake during the day as they scoured the surface of the ocean in search of fish, yet once night fell, they ceased their hunting activities and allowed themselves to recharge their batteries by catching 40 winks.
Analyzing the frigatebirds’ brain activity, the study authors found that they engaged in two types of sleep: slow wave sleep (SWS) and rapid eye movement (REM) sleep. Also known as deep sleep, SWS is characterized by very low frequency brain waves, while REM sleep involves more energetic brain activity, and is the phase of sleep associated with dreaming.
On average, the birds slept for just 12 seconds at a time, the vast majority of which was made up of SWS. The longest continuous sleep episode recorded for a bird in flight was just under six minutes.
As expected, the birds normally kept one brain hemisphere awake, especially when circling rather than flying in straight lines. Writing in the journal Nature Communications, the researchers say they believe this is so that they can keep one eye open in order to avoid crashing into other birds when turning. Intriguingly, however, both brain hemispheres were allowed to fall asleep on some occasions.
The brevity of these sleeping episodes comes as a major surprise, especially as flying frigatebirds have no predators to worry about and don’t forage at night, meaning they should be free to sleep. However, the fact that they limit their slumber so dramatically suggests that they must need to remain alert when flying at night for some unknown reason.
At present, the best explanation the researchers are able to come up with is that frigatebirds follow ocean eddies at night in order to ensure they are in the best feeding position when morning comes, so they may have to stay awake in order to avoid going astray.
Equally intriguingly, the team are unable to explain how these birds are able to function on such little sleep. In a statement, study co-author Niels Rattenborg summed up this conundrum by saying “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.”
If people got as little sleep as frigatebirds do when they are flying, we'd be pretty screwed. Stokkete/Shutterstock