Клюв и его строение у птиц и других животных.

Part of a bird

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, anuran tadpoles, monotremes (i. e. echidnas and platypuses, which have a beak like structure), sirens, pufferfish, billfishes and cephalopods. Although beaks vary significantly in size, shape, color and texture, they share a similar underlying structure. Two bony projections – the upper and lower mandibles – are covered with a thin keratinized layer of epidermis known as the rhamphotheca. In most species, two holes called nares lead to the respiratory system.

The upper mandible is supported by a three pronged bone called the intermaxillary. The upper prong of this bone is embedded into the forehead, while the two lower prongs attach to the sides of the skull. At the base of the upper mandible a thin sheet of nasal bones is attached to the skull at the nasofrontal hinge, which gives mobility to the upper mandible, allowing it to move upward and downward. In most birds, these muscles are relatively small as compared to the jaw muscles of similarly sized mammals.

The tomia (singular tomium) are the cutting edges of the two mandibles. Granivorous (seed eating) birds, for example, have ridges in their tomia, which help the bird to slice through a seed's outer hull. Most falcons have a sharp projection along the upper mandible, with a corresponding notch on the lower mandible. They use this "tooth" to sever their prey's vertebrae fatally or to rip insects apart. Some kites, principally those that prey on insects or lizards, also have one or more of these sharp projections,
as do the shrikes. The tomial teeth of falcons are underlain by bone, while the shrike tomial teeth are entirely keratinous. Some fish eating species, e. g., the mergansers, have sawtooth serrations along their tomia, which help them to keep hold of their slippery, wriggling prey. Serrations on hummingbird bills, found in 23% of all hummingbird genera, may perform a similar function, allowing the birds to effectively hold insect prey. 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. In some cases, the color of a bird's tomia can help to distinguish between similar species. The snow goose, for example, has a reddish pink bill with black tomia, while the whole beak of the similar Ross's goose is pinkish red, without darker tomia.

[[File:MeasureBeakThrush. jpg|thumb|A bird's culmen is measured in a straight line from the tip of the beak to a set point — here, where the feathering starts on the bird's forehead. and is particularly useful in feeding studies. There are several standard measurements that can be made—from the beak's tip to the point where feathering starts on the forehead, from the tip to the anterior edge of the nostrils, from the tip to the base of the skull, or from the tip to the cere (for raptors and owls)
The culmen of a juvenile common loon is all dark, while that of the very similarly plumaged juvenile yellow billed loon is pale towards the tip.

The gonys is the ventral ridge of the lower mandible, created by the junction of the bone's two rami, or lateral plates. This spot triggers begging behavior in gull chicks. The chick pecks at the spot on its parent's bill, which in turn stimulates the parent to regurgitate food.

[[File:Saker Falcon profile shot. jpg|thumb|Falcons have a small tubercule within each naris. while those over a woodpecker's nares help to keep wood particles from clogging its nasal passages. Species in the bird order Procellariformes have nostrils enclosed in double tubes which sit atop or along the sides of the upper mandible. A number of species, including the falcons, have a small bony tubercule which projects from their nares. The function of this tubercule is unknown. Some scientists suggest it may act as a baffle, slowing down or diffusing airflow into the nares (and thus allowing the bird to continue breathing without damaging its respiratory system) during high speed dives, but this theory has not been proved experimentally. Not all species that fly at high speeds have such tubercules, while some species which fly at low speeds do.

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. The shape of the beak is determined by two modules: the prenasal cartilage during early embryonic stage and the premaxillary bone during later stages. Development of the prenasal cartilage is regulated by genes Bmp4 and CaM, while that of the premaxillary bone is controlled by TGFβllr, β catenin, and Dickkopf 3. 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. β catenin is involved in the differentiation of terminal bone cells. Dickkopf 3 codes for a secreted protein also known to be expressed in mammalian craniofacial development. The combination of these signals determines beak growth along the length, depth, and width axes. Reduced expression of TGFβllr significantly decreased the depth and length of chicken embryonic beak due to the underdevelopment of the premaxillary bone. Contrarily, an increase in Bmp4 signaling would result in a reduced premaxillary bone due to the overdevelopment of the prenasal cartilage, which takes up more mesenchymal cells for cartilage, instead of bone, formation.

Some species use their beaks in displays of various sorts. As part of his courtship, for example, the male garganey touches his beak to the blue speculum feathers on his wings in a fake preening display, and the male Mandarin duck does the same with his orange sail feathers.

The beak of birds plays a role in removing skin parasites (ectoparasites) such as lice. It is mainly the tip of the beak that does this. Studies have shown that inserting a bit to stop birds from using the tip results in increased parasite loads in pigeons. It is thought that the overhang at the end of the top portion of the beak (that is the portion that begins to curve downwards) slides against the lower beak to crush parasites. Beaks with no overhang would be unable to effectively remove and kill ectoparasites as mentioned above. Studies have supported there is a selection pressure for an intermediate amount of overhang. Western Scrub Jays who had more symmetrical bills (i. e. those with less of an overhang), were found to have higher amounts of lice when tested. The same pattern has been seen in surveys of Peruvian birds. Billing can also be used as a gesture of appeasement or subordination. Subordinate Canada jay routinely bill more dominant birds, lowering their body and quivering their wings in the manner of a young bird food begging as they do so.