If you look up to the skies above Londolozi at the moment you might see a tiny bird called the Alpine swift flitting about against the blue. Imagine being this tiny bird, weighing just 77g and yet being able to fly all the way from North Africa or Europe to Londolozi on your own steam. Then when you get here, for the next 200 days, you don’t touch land once. What has bemused researchers and ornithologists for many years is how it is these birds sleep without falling out of the sky. At last, we finally have some answers!
In a paper published in Nature Communications, Niels Rattenborg from the Max Planck Institute and colleagues from several other institutions have offered the first proof showing that flying birds can sleep with either one half of their brains active, or with both hemispheres shut down at the same time. Remarkably, these birds can retain their navigational ability while in REM sleep. In other words, they can literally fly with their eyes closed.
This technique of sleeping is called unihemispheric slow-wave sleep (USWS), which allows one half of the brain to enter into deep sleep while the eye corresponding to this half is closed and the other eye remains open. Unihemispheric sleep allows an animal to get some rest, while also allowing it to maintain awareness of its surroundings. In fact, dolphins also use this technique to avoid drowning while they rest.
Rattenborg and his team did their experiment on frigatebirds, who often spend weeks flying nonstop in search of prey. What the study found was that these birds were flying upwards of 1,850 miles (3,000 kilometers) without stopping for a break. The birds would remain awake during the day while they looked for food and as the sun set they would enter into USWS for several minutes at a time.
Occasionally, the birds even entered into REM sleep. You may think this is a crazy thing to do while flying but unlike us, where bursts of REM sleep are lengthy and involve complete loss of muscle tone, REM sleep in birds lasts for only a few seconds. That said, the resulting loss of muscle tone caused the heads of the birds to dip during flight, but amazingly it doesn’t affect their flight patterns.
Despite their remarkable ability to sleep while flying, frigatebirds still get excruciatingly little sleep. On average, they sleep for only 42 minutes a day. This is similar to the giraffes we see here who only sleep for about 30 minutes every day. Unlike the giraffe who only needs minimal sleep, the frigatebirds are used to sleeping for about 12 hours a day when on land. This suggests that frigatebirds are seriously sleep deprived during their foraging flights. How they’re able to function with such little sleep remains a mystery.
Despite gaining knowledge of how Alpine swifts fly while sleeping, we still have no idea why they choose not to land during their foray down south. Not all of them do this, and some certainly do land to breed. Certain individuals however, have been shown though electronic tags in a study by the Swiss Ornithological Institute, to spend more than 6 months aloft!
Although I may be biased, South Africa’s Lowveld seems like a pretty fabulous place to spend a summer and to kick it back for a nap every now and again. The beauty of the unknown though is that it reminds us how relatively little we understand about the creatures we share the planet with, which means we can only continue to learn about the mystery we all form a part of.