Duck

Some birds like ducks, geese, and other waterfowl can be seen sleeping on or near their roosting pond, propped up on one leg, positioned in a way that the body is balanced without much additional muscle activity. To avoid predators, most ducks often seen sleeping on the water, with their head cranked around backwards and nestled into their feathers, to keep their bill warm.

Like the old adage, if you think you're in danger, you should sleep with one eye open, ducks also have a tendency of sleeping in a row in which the ducks on both ends sleep with one eye open, watching out for any dangers that lurk.