Migratory birds (候鸟)fly to different places during different seasons. They 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(军舰鸟)during a ten-day flying journey. They found that frigate birds can sleep while they're flying! According to this study, these birds take two kinds of short sleep.

The first is called Rapid Eye Movement(REM, 快速动眼)sleep. It is the deepest period of sleep. For humans, REM sleep can continue for many minutes. For flying birds, however, REM sleep continues for only a few seconds(秒). Even so, this period of sleep is still very important for their brain health. 

The other kind of short sleep is "half sleep". Frigate birds put only half of their brain to sleep, while the other half stays awake. The eyes of these birds are always connected to the waking part of their brain. In other words, they sleep with their eyes open. It is still a little dangerous to take such a short sleep, though. So, these birds prefer to stay awake most of the time. Before reaching land, they spend less than three percent of their time sleeping.

However, not all migratory birds sleep in this way. Many will make several stops on their way to get a good rest. Some stay completely awake for their whole journey. Alpine Swifts(高山雨燕), for example, can keep flying for nearly 200 days. There are few signs of these birds sleeping during their long trips. 

The sleeping habits of birds are different from species(物种) 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!