Beak
The beak, bill, or rostrum is an external anatomical structure found mostly in birds, but also in turtles, non-avian dinosaurs and a few mammals.
A beak is used for pecking, grasping, and holding (in probing for food, eating, manipulating and carrying objects, killing prey, or fighting), preening, courtship, and feeding young.
The terms beak and rostrum are also used to refer to a similar mouth part in some ornithischians, pterosaurs, cetaceans, dicynodonts, rhynchosaurs, anuran tadpoles, monotremes (i.e. echidnas and platypuses, which have a bill-like structure), sirens, pufferfish, billfishes, and cephalopods.
[4](p147) The upper, and in some cases the lower, mandibles are strengthened internally by a complex three-dimensional network of bony spicules (or trabeculae) seated in soft connective tissue and surrounded by the hard outer layers of the beak.
These ossified plates (or rami), which can be U-shaped or V-shaped,[4](p147) join distally (the exact location of the joint depends on the species) but are separated proximally, attaching on either side of the head to the quadrate bone.
[10](p598) In most birds, they range from being rounded to slightly sharp, but some species have evolved structural modifications that allow them to handle their typical food sources better.
They may also allow shorter-billed hummingbirds to function as nectar thieves, as they can more effectively hold and cut through long or waxy flower corollas.
[34] Gapes of juvenile altricial birds are often brightly coloured, sometimes with contrasting spots or other patterns, and these are believed to be an indication of their health, fitness and competitive ability.
[39] Conversely, the red gape of the common cuckoo (Cuculus canorus) did not induce extra feeding in host parents.
[10](p47) While the nares are uncovered in most species, they are covered with feathers in a few groups of birds, including grouse and ptarmigans, crows, and some woodpeckers.
[45] Species in the bird order Procellariformes have nostrils enclosed in double tubes which sit atop or along the sides of the upper mandible.
Some scientists suggest it acts as a baffle, slowing down or diffusing airflow into the nares (thus allowing the bird to continue breathing without damaging its respiratory system) during high-speed dives, but this theory has not been proved experimentally.
[50] In pigeons, the operculum has evolved into a soft swollen mass that sits at the base of the bill, above the nares;[10](p84) though it is sometimes referred to as the cere, this is a different structure.
[52] Birds from a handful of families—including raptors, owls, skuas, parrots, pigeons, turkeys and curassows—have a waxy structure called a cere (from the Latin cera, which means "wax") or ceroma[53][54] which covers the base of their bill.
[59] All birds of the family Anatidae (ducks, geese, and swans) have a nail, a plate of hard horny tissue at the tip of the beak.
Species which use strong grasping motions to secure their food (such as when catching and holding onto a large squirming frog) have very wide nails.
[70] There is some experimental evidence to suggest that they may prevent particles from striking the eyes if, for example, a prey item is missed or broken apart on contact.
Studies have shown that Herbst corpuscles, mechanoreceptors sensitive to pressure and vibration, are found in association with rictal bristles.
Then it pecks at the eggshell while turning slowly within the egg, eventually (over a period of hours or days) creating a series of small circular fractures in the shell.
[82] Many hornbills show sexual dimorphism in the size and shape of both beaks and casques, and the female huia's slim, decurved bill was nearly twice as long as the male's straight, thicker one.
Castration also prevents the normal seasonal color change in the beaks of male black-headed gulls and indigo buntings.
[83] The beak of modern birds has a fused premaxillary bone, which is modulated by the expression of Fgf8 gene in the frontonasal ectodermal zone during embryonic development.
[85][86] TGFβllr codes for a serine/threonine protein kinase that regulates gene transcription upon ligand binding; previous work has highlighted its role in mammalian craniofacial skeletal development.
Reduced expression of TGFβllr significantly decreased the depth and length of chicken embryonic beak due to the underdevelopment of the premaxillary bone.
Studies have shown that inserting a bit to stop birds from using the tip results in increased parasite loads in pigeons.
[93] A number of species, including storks, some owls, frogmouths and the noisy miner, use bill clapping as a form of communication.
[77](p83) Some woodpecker species are known to use percussion as a courtship activity, whereas males get the (aural) attention of females from a distance and then impress them with the sound volume and pattern.
By dumping excess heat through their bills, the sparrows are able to avoid the water loss which would be required by evaporative cooling—an important benefit in a windy habitat where freshwater is scarce.
[103] Alternately, studies have shown that birds from colder climates (higher altitudes or latitudes and lower environmental temperatures) have smaller beaks, lessening heat loss from that structure.
[114] A similar but separate practice, usually performed by an avian veterinarian or an experienced birdkeeper, involves clipping, filing or sanding the beaks of captive birds for health purposes–in order to correct or temporarily alleviate overgrowths or deformities and better allow the bird to go about its normal feeding and preening activities.
