The beak, bill or rostrum is an external anatomical structure of birds which, in addition to eating, is used for grooming, manipulating objects, killing prey, probing for food, courtship and feeding their young. The term also refers to a similar mouthpart in some cephalopods, cetaceans, pufferfishes, turtles, Anuran tadpoles and sirens.

Anatomy

Beaks can vary significantly in size and shape from species to species. The beak is composed of an upper jaw called the maxilla, and a lower jaw called the mandible. The jaw is made of bone, typically hollow or porous to conserve weight for flying. The outside surface of the beak is covered by a thin horny sheath of keratin called the rhamphotheca. Between the hard outer layer and the bone is a vascular layer containing blood vessels and nerve endings. The rhamphotheca also includes the knob which is found above the beak of some swans, such as the Mute Swan, and some domesticated Chinese geese (pictured).

The beak has two holes called nares (nostrils) which connect to the hollow inner beak and thence to the respiratory system. The nares are usually located directly above the beak. In some birds, they are located in a fleshy, often waxy structure at the base of the beak called the cere (from Latin cera). Hawks, parrots, doves, skuas and budgerigars are among the birds that have ceres. Budgerigars are dimorphic in that the males' ceres turn bright blue upon maturity, while the females' ceres turn tan. The female budgies' ceres also appear wrinkled, to a greater extent during periods of fertility. Immature budgies have pale pinkish ceres which are smooth and shiny.

On some birds, the tip of the beak is hard, dead tissue used for heavy-duty tasks such as cracking nuts or killing prey. On other birds, such as ducks, the tip of the bill is sensitive and contains nerves, for locating things by touch. The beak is worn down by use, so it grows continuously throughout the bird's life.

Unlike jaws with teeth, beaks are not used for chewing. Birds swallow their food whole, which is broken up in the gizzard.

Examples of birds with unusual beaks include the hummingbird, the toucan and the spoonbill.

In the mallard, and perhaps in other ducks, there is no cere, and the nostrils are in the hard part of the beak, as a soft cere would be liable to injury when the duck dredges for food among submerged debris and stones.

Billing

During courtship, mated pairs of a variety of bird species touch and clasp each other's bills. This is called billing, and appears to strengthen the pair bond (Terres, 1980). Gannets raise their bills high and repeatedly clatter them (pictured); the male puffin nibbles at the female's beak; the male waxwing puts his bill in the female's mouth; and ravens hold each other's beaks in a prolonged "kiss".