Why do some birds sleep with one eye open?

Many birds have a sleep strategy that allows part of the brain to rest while the other part remains watchful. This is called unihemispheric slow-wave sleep, and it is especially useful in risky environments or during flock resting. Birds can keep one eye open and maintain some awareness of danger while still getting essential sleep. Waterfowl and other species often show this behavior when sleeping in groups, with birds at the edge of the flock more alert. It is a clever compromise between rest and survival.