Duck and Sleep.

If you see a duck's head tucked in its wing, one eye closed, one eye opened, believe it or not, it's asleep.

Duck's brains are split in half, controlling one eye and the other half controlling the other eye. So it is entirely possible for them to "turn off" half their brain to rest it while the other half remains alert for predators. Ducks only fully rest both halves if they are in a large group where others can be on the lookout.