Migratory birds fly to different places during different seasons. This is bird migration. Birds in migration may fly day and night for months at a time. During this long journey, when and how do they sleep?
In 2016, some scientists studied a group of frigate birds during a ten-day flight. They recorded these birds' brain activities. Their findings made headlines-frigate birds can sleep while they are flying!
According to this study, these birds take two types of in-flight naps. The first is called Rapid Eye Movement （REM）sleep. It is the deepest stage of sleep. For humans, REM sleep can last for several minutes. For flying birds, however, REM sleep lasts for only a few seconds. Even so, REM sleep is still very important for proper brain functioning.
The other type of nap frigate birds take is Non-Rapid Eye Movement （NREM） sleep. During NREM sleep, the birds put only half of their brain to sleep, while the other half stays awake. The eyes of these birds are connected to the waking part of their brain. In other words, they sleep with their eyes open. They do this to stay away from dangers, and many birds do this even when they aren't migrating.
Even though frigate birds sleep during the flight, they prefer to stay awake most of the time, spending less than 3% of their time sleeping. Once they reach land, they recover their lost sleep, spending almost half their time dozing.
However, not all migratory birds sleep in this way. Many make occasional pit stops on their way to get a good rest before the next leg of their journey. Some stay completely awake during the whole journey. The Alpine Swift, for example, can fly for almost 200 days continuously. There is still little evidence of these birds sleeping during their long journeys.
The sleeping habits of birds differ from species to species. This makes the study of migratory birds very exciting for scientists. In the future, we may learn more amazing things about these animals.
