beak

The beak, bill or rostrum is an external anatomical structure of birds which is used for eating and for grooming, manipulating objects, killing prey, probing for food, courtship and feeding young. The term also refers to a similar mouthpart in some monotremes, cephalopods, cetaceans, pufferfishes, turtles, Anuran tadpoles and sirens.
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.
Maxilla, mandible
The maxilla (plural: maxillae), also known as the mustache bone, is a fusion of two bones along the palatal fissure that form the upper jaw. This is similar to the mandible (lower jaw), which is also a fusion of two halves at the mental symphysis. Sometimes (e.g. in bony fish), the maxilla is sometimes called "upper maxilla", with the mandible being the "lower maxilla". Conversely, in birds the upper jaw is often called "upper mandible".
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).
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 continually throughout the bird's life.
As noted by Darwin in his observations on Galapagos Finches, birds' beaks have evolved to suit the ecological niche they fill: Raptors have decurved (downward curving) beaks for ripping up meat. Hummingbirds have long thin beaks for reaching nectar. The spoonbills' beaks allow them to filter-feed in shallow water. Unlike jaws with teeth, beaks are not used for chewing. Birds swallow their food whole, and it is broken up in the gizzard.

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