Beak>

The terms 'beak' and 'bill' are interchangeable, although the former was formerly restricted to hooked beaks of birds of prey and parrots.

Beaks 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 can include 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 at the base of the beak, near the dorsal surface. Kiwi are the only birds with nostrils at the end of their beak. In some birds, the nares are in a fleshy, often waxy structure at the base of the beak called the cere (from Latin cera, meaning wax). The cere is an indicator of the reproductive cycle of budgerigars.

Petrels and albatrosses have external horny sheaths called naricorns that protect the nares. These are separately placed on either side of the base of the upper mandible in albatrosses, but fused, with an internal septum, on the top of the base of the upper mandible in petrels.. 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.

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