There are many birds around the world that are known for flying non-stop for days and even weeks. This phenomenon has stumped scientists for decades now, but due to the lack of studies monitoring the sleep patterns of flying birds, getting to the bottom of this mystery was quite difficult. One of the hypotheses explaining this phenomenon was that the birds sleep in mid-flight, hence they are able to fly for days without any rest. But this was always countered by the argument that sleep deprivation barely affects certain species, hence this wasn’t plausible. That was the case until recently, when a new study from the Max Planck Institute for Ornithology researchers finally found concrete evidence suggesting that birds do indeed sleep while flying.
The team was being led by a neurophysiologist Niels Rattenborg, and he and his team authored the study made in the Galápagos Islands, where they monitored the brain activity of great frigatebirds (Fregata minor). This bird is a species of large seabird well known for its amazing ability to spend weeks flying non-stop over the ocean.
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The research was done by attaching a small device to the heads of the frigate birds, and then waiting for the birds to come back after the flight. The device used electroencephalography (EEG) so that it could identify and record when the bird went to sleep while they flew over the ocean. They picked out one bird while it was still on land, attached the device on its head and after about 10 days they finally caught the bird back and recollected the device to observe the results.
The team found out that the flying frigatebirds exhibited unihemispheric slow wave sleep (USWS), which is a fascinating phenomenon where animals sleep with only one hemisphere of the brain at a time. This allows them be able to sleep with only one eye closed, while the other eye is open and ready to watch out for potential threats. Many other birds like the mallard duck use USWS while being on land. Mammals like Dolphins have also been observed exhibiting USWS which lets them sleep while they are still swimming.
Amazingly this is the first time that birds have been observed sleeping mid-flight.
But one more surprising discovery made was that the frigatebirds can exhibit bihemispheric sleep, in which both hemispheres of the brain are asleep at the same time. This recorded data meant that frigatebirds were flying with both of their eyes closed while the device was attached to its head, which sounds astounding. These birds even experienced brief moments of rapid eye movement (REM) sleep, although that only lasted for a few seconds. Despite the REM sleep where the muscle tone reduces and the head drops, it did not affect the birds’ flight patterns one bit.
But it was also recorded that the majority of the flight was spent awake. On land, these frigatebirds can sleep for over 12 hours per day, but it is quite astonishing to realize that it can perform so well while sleeping for just about 42 minutes per day while flying.
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