Bird beaks, or bills, come in many shapes and sizes. And birds use them for just about everything: to collect food, preen, fight, court, chop holes in trees, weave nests, and more. In order for a bird to fly, its beak must weigh as little as possible. Beaks are covered with a sheath of a tough material called keratin, which grows continuously because a beak wears down with use. Learn more at BirdNote.org.