There's an old saying that if you think you're in danger, you might ought to sleep with one eye open.
Well it turns out that many kinds of birds do just that.
Row Of Ducks
A group of scientists at Indiana State University studied mallard ducks to learn more about this trait. They filmed a row of ducks while they were sleeping. Sure enough they found that a good part of the time the ducks at the ends of the row kept the eye facing away from the group open, while the eye toward the other ducks closed in sleep. The ducks within the row were more likely to close both of their eyes to sleep.
The ducks at the ends of the line were actually controlling which side of the brain stayed awake, and which dozed. They were engaging in what scientists call single-hemisphere sleep. The eye controlled by the sleeping side of the brain closed, while the active side kept its eye open and on the lookout.
You can see how this is good for the group and for the lookout duck as well. The group gets the protection from having someone pulling guard duty, while the guard duck is at least able to get some rest.