A bony extension of a bird’s skull, the beak (also called a bill or rostrum) includes an upper and lower jaw covered by a horny, proteinaceous sheath.
The beak is used for eating and for grooming, manipulating objects, killing prey, probing for food, courtship and feeding young. The term may also refer to a similar mouthpart in some other kinds of organisms.
Definition adapted from URL: http://en.wikipedia.org/wiki/Beak