The Greater frigatebird, a hunter of the open ocean, sometimes spends months on the wing, riding the winds and fishing far offshore. The Alpine swift has been recorded spending 200 days in the air, only returning to the ground to breed, and migrating birds often fly non-stop for days while crossing oceans and deserts.
Not only are these amazing feats of endurance, but they also raise a logical question. How do these birds get any sleep, or do they sleep at all?
The answer is that they do sleep, but not in ways familiar to humans, but by utilising a process called uinhemispheric slow-wave sleep (USWS). This allows them to keep half their brains awake and alert but at the same time allowing the other side to sleep (or lapse into shallow sleep). They also utilise regular “micronaps,” which only last a few seconds.
Scientists have known for decades that marine mammals such as dolphins, whales, and seals use USWS but it was first recorded in birds which were observed sleeping with one eye open, quite literally, which allows them to spot impending danger. The birds can select which side of the brain they want to sleep.
Some marine mammals use the same technique to ensure that they continue to swim and breathe, all the while managing to allow their brains to sleep, a process which any sleep-deprived human knows, is critical to health and mental wellbeing.
Although aware that birds regularly use USWS while roosting on land, researcher Niels Rattenborg and his colleagues from the Max Planck Institute for Ornithology were the first to prove that the technique is also used in the air. Using small electronic devices which monitored the brain activity of Greater frigatebirds, large seabirds with two metre wingspans, Rattenborg concluded that “they can sleep in flight with one hemisphere of the brain) at a time or both together.”
“Although frigatebirds engage in both types of sleep (USWS and conventional) on the wing, sleep is more asymmetric in flight than when on land,” Rattenborg wrote in the scientific publication Nature Communications.
Frigatebirds sleep mostly while circling in rising air currents and keep the eye connected to the awake hemisphere facing the direction of flight, suggesting that they use uinhemispheric sleep to watch where they are going.
Frigatebirds and some long-distance migrant birds also engage in microsleeps”, which last only a few seconds at a time, but may be taken thousands of times a day. Although the Frigatebirds birds coped for months using these techniques they slept for longer than normal periods once they returned to land, seemingly in an effort catch up on “quality” sleep.
Frigatebirds cannot rest on the ocean surface, they become waterlogged and drown, so the use of these sleep techniques is an ecological trade-off because it has been shown that birds sleep more efficiently when both sides of the brain are closed down.