Beak
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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.
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[edit] 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 Swan Geese (pictured).
The beak has two holes called nares which connect to the hollow inner beak and thence to the respiratory system. In some birds, these are located in a fleshy, often waxy structure at the base of the beak called the cere (from Latin cera). Hawks, parrots, doves, and skuas are among the birds that have ceres. Budgerigars are dimorphic because 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.
Nares are bird nostrils. The nares of birds are usually located directly above the beak. On some birds, such as the budgerigar, the nares are situated within the cere.
Nares can also refer to nostrils on other animals, such as sharks, rays, and sawfishes. Nares is a medical term from Latin that describes human nostrils.
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.
[edit] 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".
[edit] See also
[edit] References
- Gilbertson, Lance; Zoology Lab Manual; McGraw Hill Companies, New York; ISBN 0-07-237716-X (fourth edition, 1999)
- Terres, John. K. The Audubon Society Encyclopedia of North American Birds, New York: Alfred A. Knopf, 1980. ISBN 0-394-46651-9
