thumb|Comparison of bird beaks, displaying different shapes adapted to different feeding methods; not to scale
The beak or bill is an external rostrum structure found mostly in birds. 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.
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.
Etymology
Although the word "beak" was, in the past, generally restricted to the sharpened bills of birds of prey,
Mandibles
thumb|A [[gull's upper mandible can flex upwards because it is supported by small bones which can move slightly backwards and forwards.]]
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.
Rhamphotheca
thumb|[[Mallard with an injured beak after a dog attack. Parts of the rhamphotheca have detached, exposing the periosteum.]]
The outer surface of the beak consists of a thin sheath of keratin called the rhamphotheca, There is a vascular layer between the rhamphotheca and the deeper layers of the dermis, which is attached directly to the periosteum of the bones of the beak. The rhamphotheca grows continuously in most birds, and in some species, the color varies seasonally.
In some alcids, such as the puffins, parts of the rhamphotheca are shed each year after the breeding season, while some pelicans shed a part of the bill called a "bill horn" that develops in the breeding season.
While most extant birds have a single seamless rhamphotheca, species in a few families, including the albatrosses
Studies have shown that this was the primitive ancestral state of the rhamphotheca, and that the modern simple rhamphotheca resulted from the gradual loss of the defining grooves through evolution.
Tomia
thumb|right|The sawtooth serrations on a [[common merganser's bill help it to hold tight to its prey, a fish.]]
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.
Culmen
[[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 which 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.
Gonys
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.
Commissure
Depending on its use, commissure may refer to the junction of the upper and lower mandibles,
The width of the gape can be a factor in the choice of food.
thumb|left|The gape flange on this juvenile [[house sparrow is the yellowish region at the base of the beak.]]
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. Based on that, the parents decide how to distribute food among the chicks in the nest.
Some species, especially in the families Viduidae and Estrildidae, have bright spots on the gape known as gape tubercles or gape papillae. These nodular spots are conspicuous even in low light.
A study examining the nestling gapes of eight passerine species found that the gapes were conspicuous in the ultraviolet spectrum (visible to birds but not to humans).
Parents may, however, not rely solely on the gape coloration, and other factors influencing their decision remain unknown.
Red gape color has been shown in several experiments to induce feeding. An experiment in manipulating brood size and immune system with barn swallow nestlings showed the vividness of the gape was positively correlated with T-cell–mediated immunocompetence, and that larger brood size and injection with an antigen led to a less vivid gape.
Conversely, the red gape of the common cuckoo (Cuculus canorus) did not induce extra feeding in host parents.
Some brood parasites, such as the Hodgson's hawk-cuckoo (C. fugax), have colored patches on the wing that mimic the gape color of the parasitized species.
When born, the chick's gape flanges are fleshy. As it grows into a fledgling, the gape flanges remain somewhat swollen and can thus be used to recognize that a particular bird is young.
By the time it reaches adulthood, the gape flanges will no longer be visible.
Nares
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 Procellariiformes 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 the tubercule is unknown. 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. Not all species which fly at high speeds have these tubercules, while some species which fly at low speeds have them.
Operculum
thumb| The [[rock dove's operculum is a mass at the base of the bill.]]
The nares of some birds are covered by an operculum (plural opercula), a membraneous, horny or cartilaginous flap.
Some species which feed on flowers have opercula to help to keep pollen from clogging their nasal passages,
The nares of nestling tawny frogmouths are covered with large dome-shaped opercula, which help to reduce the rapid evaporation of water vapor, and may also help to increase condensation within the nostrils themselves—both critical functions, since the nestlings get fluids only from the food their parents bring them. The opercula shrink as the birds age, disappearing completely by the time they reach adulthood.
In pigeons, the operculum has evolved into a soft swollen mass that sits at the base of the bill, above the nares;
at the corners of the beak. In the puffin, it is grown as part of its display plumage.
which covers the base of their bill. This structure typically contains the nares, except in the owls, where the nares are distal to the cere. Although it is sometimes feathered in parrots,
the cere is typically bare and often brightly colored.
The cere color of young Eurasian scops-owls has an ultraviolet (UV) component, with a UV peak that correlates to the bird's mass. A chick with a lower body mass has a UV peak at a higher wavelength than a chick with a higher body mass does. Studies have shown that parent owls preferentially feed chicks with ceres that show higher wavelength UV peaks, that is, lighter-weight chicks.
The color or appearance of the cere can be used to distinguish between males and females in some species. For example, the male great curassow has a yellow cere, which the female (and young males) lack.
The male budgerigar's cere is royal blue, while the female's is a very pale blue, white, or brown.
Nail
thumb|The black tip of this [[mute swan's beak is its nail.]]
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. The shield-shaped structure, which sometimes spans the entire width of the beak, is often bent at the tip to form a hook.
It serves different purposes depending on the bird's primary food source. Most species use their nails to dig seeds out of mud or vegetation,
while diving ducks use theirs to pry molluscs from rocks.
There is evidence that the nail may help a bird to grasp objects. 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.
Certain types of mechanoreceptors, nerve cells that are sensitive to pressure, vibration, or touch, are located under the nail.
The shape or color of the nail can sometimes be used to help distinguish between similar-looking species or between various ages of waterfowl. For example, the greater scaup has a wider black nail than does the very similar lesser scaup.
Juvenile "grey geese" have dark nails, while most adults have pale nails.
The nail gave the wildfowl family one of its former names: "Unguirostres" comes from the Latin ungus, meaning "nail" and rostrum, meaning "beak".
They are common among insectivorous birds, but are also found in some non-insectivorous species.
Their function is uncertain, although several possibilities have been proposed.
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.
Despite its name, the projection is not an actual tooth, as the similarly-named projections of some reptiles are; instead, it is part of the integumentary system, as are claws and scales.
The hatching chick first uses its egg tooth to break the membrane around an air chamber at the wide end of the egg. 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.
Most chicks lose their egg teeth within a few days of hatching,
Eumelanin, which is found in the bare parts of many bird species, is responsible for all shades of gray and black; the denser the deposits of pigment found in the epidermis, the darker the resulting color. Phaeomelanin produces "earth tones" ranging from gold and rufous to various shades of brown.
King and emperor penguins, for example, show spots of ultraviolet reflectance only as adults. These spots are brighter on paired birds than on courting birds. The position of such spots on the beak may be important in allowing birds to identify conspecifics. For instance, the very similarly-plumaged king and emperor penguins have UV-reflective spots in different positions on their beaks.
For example, females of nearly all shorebirds have longer bills than males of the same species,
and female American avocets have beaks which are slightly more upturned than those of males.
Males of the larger gull species have bigger, stouter beaks than those of females of the same species, and immatures can have smaller, more slender beaks than those of adults.
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.
Development
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.
Displays (for courtship, territoriality, or deterrence)
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 beaks of Kiwis, Ibises, and sandpipers have sensory pits in their beaks that allow them to sense vibrations.
The beaks of aquatic birds contain Grandry corpuscles, which assist in velocity detection while filter feeding.
Preening
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 found 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 begging for food as they do so.
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