Dr Rattenborg, can birds sleep on the wing?

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“Yes, birds can indeed sleep on the wing. We proved that this is the case using the example of the frigatebird. These birds live mainly in the tropics and subtropics. The males have a distinctive red gular pouch that they can inflate like a balloon. Perhaps you are wondering why we chose to study these birds in particular. Frigatebirds spend more than a week flying continuously over the ocean in search of their prey. So it is natural to question whether the birds in fact need any sleep at all during this time. Just imagine you spent that much time working without any sleep! Not only is this inconceivable, it is also impossible.

Some birds go without sleep for days on end

a frigatebird near the Galapagos Islands

For more than a century, researchers have been arguing about whether birds sleep on the wing or whether they perhaps do not sleep at all. As a matter of fact, this is precisely what the pectoral sandpiper (Calidris melanotos) does. These birds, which live in Alaska, get virtually no sleep at all when they are breeding. That said, they are not continuously in the air during this time. We wanted to find out whether frigatebirds also do not sleep at all – even when they are spending whole weeks on the wing. To this end, we flew to the Galapagos Islands, which are home to many of these birds. Researchers at the University of Zurich and the ETH Zurich developed a data logger for us in advance. It is used to measure movements of the head and brain activity. We can identify the birds’ waking and sleep phases by monitoring the activity of their brains. We attached the device to 14 birds using a special adhesive – the same substance that is used to seal sutures in humans. We additionally fitted a GPS tracker to the backs of the birds.

Frigatebirds indulge in a “power nap”

Once the birds returned to land, we analysed the data and were amazed at what we found. The readings showed that frigatebirds do sleep on the wing – and even do so in different ways. While searching for food during the day, they remain awake the entire time. They then use the night-time for a variety of sleep phases, which on average last only 42 minutes in total. The longest period of uninterrupted sleep that we measured lasted just under six minutes. For the majority of this time, the birds sleep with only one half of the brain.

Why it is often only one half of the brain that sleeps

Some time ago, I already observed a similar phenomenon in ducks. Although they generally sleep with both halves of the brain, birds at the edge of a group keep one eye open – the eye that is looking away from the group. This allows them to keep watch for any potential predators. The corresponding half of the brain is awake, while the half associated with the closed eye is asleep. Dolphins can also swim while one half of their brain is asleep so that they can keep moving at the same time. While frigatebirds are circling and taking advantage of rising air currents, one half of their brain remains awake, as they need to keep one eye open to avoid colliding with other birds. There is no such risk when the birds are gliding, so during this phase – which only lasts for a few minutes per night, however – the frigatebirds even sleep with both halves of the brain.

Researchers hope to learn more about sleep deficit in humans

The really fascinating thing is that the frigatebirds spend an average of twelve hours a day asleep once they are back on land – far more than in the air, in other words. As yet, we still do not know why they sleep so little on the wing or how they cope with the lack of sleep. We hope to be able to answer these questions in the future. We also hope that our research will give us new insights into the consequences of sleep deficit in other animals – in humans for instance.”

Thank you for this fascinating answer, Dr Rattenborg.