Beak

Beak

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The beak, bill or rostrum is an external anatomical structure of birds which is used for eating and for grooming, manipulating objects, killing prey, fighting, probing for food, courtship and feeding young. The terms beak and rostrum
Rostrum (anatomy)
The term rostrum is used for a number of unrelated structures in different groups of animals:*In crustaceans, the rostrum is the forward extension of the carapace in front of the eyes....

are also used to refer to a similar mouthpart in some Ornithischian dinosaurs, monotremes, cephalopod
Cephalopod
A cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...

s, cetaceans, pufferfishes, turtle
Turtle
Turtles are reptiles of the order Testudines , characterised by a special bony or cartilaginous shell developed from their ribs that acts as a shield...

s, Anuran tadpole
Tadpole
A tadpole or polliwog is the wholly aquatic larval stage in the life cycle of an amphibian, particularly that of a frog or toad.- Appellation :...

s and sirens.

Although beaks can vary significantly in size, shape and color, 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 known as nares lead to the respiratory system.

Etymology


Although the distinction between 'beak' and 'bill' is blurry, the former is generally restricted to the sharpened bills of birds of prey that can tear flesh, whereas 'bill' is a more general term referring to any bird rostrum.

Anatomy



Although beaks vary significantly in size and shape from species to species, their underlying structures have a similar pattern. All beaks are composed of two jaws, generally known as the upper mandible (or maxilla) and lower mandible (or mandible). Both jaws are strengthened internally by a complex three-dimensional
Three-dimensional space
Three-dimensional space is a geometric 3-parameters model of the physical universe in which we live. These three dimensions are commonly called length, width, and depth , although any three directions can be chosen, provided that they do not lie in the same plane.In physics and mathematics, a...

 network of bony spicules (or trabecula
Trabecula
A trabecula is a small, often microscopic, tissue element in the form of a small beam, strut or rod, generally having a mechanical function, and usually composed of dense collagenous tissue They can be composed of other materials; in the heart, for example, muscles such as trabeculae carneae...

e) seated in soft connective tissue
Connective tissue
"Connective tissue" is a fibrous tissue. It is one of the four traditional classes of tissues . Connective Tissue is found throughout the body.In fact the whole framework of the skeleton and the different specialized connective tissues from the crown of the head to the toes determine the form of...

 and surrounded by the hard outer layers of the beak.

Bones of the upper mandible



The upper mandible is supported by a three-pronged bone
Bone
Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...

 called the intermaxillary or premaxillary. The upper prong of this bone is embedded into the forehead, while the two lower prongs attach to the sides of the skull
Skull
The skull is a bony structure in the head of many animals that supports the structures of the face and forms a cavity for the brain.The skull is composed of two parts: the cranium and the mandible. A skull without a mandible is only a cranium. Animals that have skulls are called craniates...

. 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 upwards and downwards.

The base of the upper mandible or the roof when seen from the mouth is the palate, the structure of which differs greatly in the ratites. Here the vomer
Vomer
The vomer is one of the unpaired facial bones of the skull. It is located in the midsagittal line, and articulates with the sphenoid, the ethmoid, the left and right palatine bones, and the left and right maxillary bones.-Biology:...

 is large large and connects with premaxillae and maxillopalatine bones in a condition termed as a "paleognathous palate". All other extant birds have a narrow forked vomer that does not connect with other bones and is then termed as neognathous. The shape of these bones varies across the bird families.

Bones of the lower mandible


The lower mandible is supported by a bone known as the inferior maxillary bone—a compound bone composed of two distinct ossified pieces. These ossified plates (or rami), which can be U-shaped or V-shaped, join distally (the exact location of the join depends on the species) but are separated proximally, attaching on either side of the head to the quadrate bone. The jaw muscles, which allow the bird to close its beak, attach to the proximal end of the lower mandible and to the bird's skull. The muscles that depress the lower mandible are usually weak except in a few birds such as the starlings and the extinct Huia
Huia
The Huia was the largest species of New Zealand wattlebird and was endemic to the North Island of New Zealand. Its extinction in the early 20th century had two primary causes. The first was rampant overhunting to procure Huia skins for mounted specimens, which were in worldwide demand by...

 which have well developed digastric muscle
Digastric muscle
The digastric muscle is a small muscle located under the jaw. so digastric muscles are muscle fibers in ligament of treitz ,omohyoid , occipitofrontalis....

s that aid in foraging by prying or gaping actions. In most birds, these muscles are relatively small as compared to the jaw muscles of similarly sized mammals.

Rhamphotheca


The outer surface of the beak consists a thin horny sheath of keratin
Keratin
Keratin refers to a family of fibrous structural proteins. Keratin is the key of structural material making up the outer layer of human skin. It is also the key structural component of hair and nails...

 called the rhamphotheca, which can be subdivided into the rhinotheca of the upper mandible and the gnathotheca of the lower mandible. This covering arises from the Malpighian cells of the bird's epidermis
Epidermis (zoology)
The Epidermis is an epithelium that covers the body of an eumetazoan . Eumetazoa have a cavity lined with a similar epithelium, the gastrodermis, which forms a boundary with the epidermis at the mouth.Sponges have no epithelium, and therefore no epidermis or gastrodermis...

, growing from plates at the base of each mandible. There is a vascular
Vascular
Vascular in zoology and medicine means "related to blood vessels", which are part of the circulatory system. An organ or tissue that is vascularized is heavily endowed with blood vessels and thus richly supplied with blood....

 layer between the rhamphotheca and the deeper layers of the dermis, which is attached directly to the periosteum
Periosteum
Periosteum is a membrane that lines the outer surface of all bones, except at the joints of long bones. Endosteum lines the inner surface of all bones....

 of the bones of the beak. The rhamphotheca grows continuously in most birds and in some like the Common Starling the colour 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 albatross
Albatross
Albatrosses, of the biological family Diomedeidae, are large seabirds allied to the procellariids, storm-petrels and diving-petrels in the order Procellariiformes . They range widely in the Southern Ocean and the North Pacific...

es and the Emu
Emu
The Emu Dromaius novaehollandiae) is the largest bird native to Australia and the only extant member of the genus Dromaius. It is the second-largest extant bird in the world by height, after its ratite relative, the ostrich. There are three subspecies of Emus in Australia...

, have compound rhamphothecae that consist of several pieces separated and defined by softer keratinous grooves. 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
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...

.

Tomia



The tomia (singular "tomium") are the cutting edges of the two mandibles. In most birds, these range from rounded to slightly sharp, but some species have evolved structural modifications which allow them to better handle their typical food sources. Granivorous (seed-eating) birds, for example, have ridges in their tomia which help the bird to slice through a seed's outer hull
Husk
Husk in botany is the outer shell or coating of a seed. It often refers to the leafy outer covering of an ear of maize as it grows on the plant. Literally, a husk or hull includes the protective outer covering of a seed, fruit or vegetable...

. Most falcon
Falcon
A falcon is any species of raptor in the genus Falco. The genus contains 37 species, widely distributed throughout Europe, Asia, and North America....

s have a sharp projection along the upper mandible, with a corresponding notch on the lower mandible. They use this "tooth" to fatally sever their prey's vertebrae or to rip insects apart. Some kites
Kite (bird)
Kites are raptors with long wings and weak legs which spend a great deal of time soaring. Most feed mainly on carrion but some take various amounts of live prey.They are birds of prey which, along with hawks and eagles, are from the family Accipitridae....

, principally those that prey on insects or lizards, also have one or more of these sharp projections, as do the shrike
Shrike
Shrikes are passerine birds of the family Laniidae. The family is composed of thirty-one species in three genera. The family name, and that of the largest genus, Lanius, is derived from the Latin word for "butcher", and some shrikes were also known as "butcher birds" because of their feeding habits...

s. Some fish-eating species, like the mergansers, have sawtooth serrations along their tomia which help them to keep hold of their slippery, wriggling prey.

Birds in roughly 30 families have tomia lined with tight bunches of very short bristles along their entire length. Most of these species are either insectivorous (preferring hard-shelled prey) or snail
Snail
Snail is a common name applied to most of the members of the molluscan class Gastropoda that have coiled shells in the adult stage. When the word is used in its most general sense, it includes sea snails, land snails and freshwater snails. The word snail without any qualifier is however more often...

 eaters, and the brush-like projections may help to increase the coefficient of friction between the mandibles, thereby improving the bird's ability to hold hard prey items. Serrations on hummingbird
Hummingbird
Hummingbirds are birds that comprise the family Trochilidae. They are among the smallest of birds, most species measuring in the 7.5–13 cm range. Indeed, the smallest extant bird species is a hummingbird, the 5-cm Bee Hummingbird. They can hover in mid-air by rapidly flapping their wings...

 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
Snow Goose
The Snow Goose , also known as the Blue Goose, is a North American species of goose. Its name derives from the typically white plumage. The genus of this bird is disputed...

, for example, has a reddish-pink bill with black tomia, while the whole beak of the similar Ross's Goose
Ross's Goose
The Ross's Goose is a North American species of goose.The American Ornithologists' Union places this species and the other two "white" geese in the genus Chen rather than the more traditional "grey" goose genus Anser.This goose breeds in northern Canada, mainly in the Queen Maud Gulf Migratory...

 is pinkish-red, without darker tomia.

Culmen



The culmen is the dorsal ridge of the upper mandible. Likened by ornithologist Elliott Coues
Elliott Coues
Elliott Coues was an American army surgeon, historian, ornithologist and author.Coues was born in Portsmouth, New Hampshire. He graduated at Columbian University, Washington, D.C., in 1861, and at the Medical school of that institution in 1863...

 to the ridge line of a roof, it is the "highest middle lengthwise line of the bill" and runs from the point where the upper mandible emerges from the forehead's feathers to its tip. The bill's length along the culmen is one of the regular measurements made during bird banding (ringing)
Bird ringing
Bird ringing or bird banding is a technique used in the study of wild birds, by attaching a small, individually numbered, metal or plastic tag to their legs or wings, so that various aspects of the bird's life can be studied by the ability to re-find the same individual later...

, 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)—and scientists from various parts of the world generally favor one method over another. In all cases, these are chord measurements
Chord (geometry)
A chord of a circle is a geometric line segment whose endpoints both lie on the circumference of the circle.A secant or a secant line is the line extension of a chord. More generally, a chord is a line segment joining two points on any curve, such as but not limited to an ellipse...

 (measured in a straight line from point to point, ignoring any curve in the culmen) taken with calipers.

The shape or color of the culmen can also help with the identification of birds in the field. For example, the culmen of the Parrot Crossbill
Parrot Crossbill
The Parrot Crossbill is a small passerine bird in the finch family Fringillidae.This bird breeds in the pine forests of northwest Europe and into western Russia...

 is strongly decurved, while that of the very similar Common Crossbill
Common Crossbill
The Common Crossbill is a small passerine bird in the finch family Fringillidae. It breeds in the spruce forests of North America, where it is known as Red Crossbill, as well as Europe and Asia; some populations breed in pine forests in certain areas of all three continents, and in North...

 is more moderately curved. The culmen of a juvenile Great Northern Loon is all dark, while that of the very similarly plumage
Plumage
Plumage refers both to the layer of feathers that cover a bird and the pattern, colour, and arrangement of those feathers. The pattern and colours of plumage vary between species and subspecies and can also vary between different age classes, sexes, and season. Within species there can also be a...

d juvenile Yellow-billed Loon
Yellow-billed Loon
The Yellow-billed Loon , also known as the White-billed Diver, is the largest member of the loon or diver family. Breeding adults have a black head, white underparts and chequered black-and-white mantle. Non-breeding plumage is drabber with the chin and foreneck white...

 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. The proximal end of that junction—where the two plates separate—is known as the gonydeal angle or gonydeal expansion. In some gull species, the plates expand slightly at that point, creating a noticeable bulge; the size and shape of the gonydeal angle can be useful in identifying between otherwise similar species. Adults of many species of large gulls have a reddish or orangish gonydeal spot near the gonydeal expansion. 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 usage, commissure may refer to the junction of the upper and lower mandibles, or alternately, to the full-length apposition of the closed mandibles, from the corners of the mouth to the tip of the beak.

Nares



Most species of birds have external nares (nostril
Nostril
A nostril is one of the two channels of the nose, from the point where they bifurcate to the external opening. In birds and mammals, they contain branched bones or cartilages called turbinates, whose function is to warm air on inhalation and remove moisture on exhalation...

s) located somewhere on their beak. The nares are two holes—circular, oval or slit-like in shape—which lead to the nasal cavities within the bird's skull, and thus to the rest of the respiratory system. In most bird species, the nares are located in the basal third of the upper mandible. Kiwi
Kiwi
Kiwi are flightless birds endemic to New Zealand, in the genus Apteryx and family Apterygidae.At around the size of a domestic chicken, kiwi are by far the smallest living ratites and lay the largest egg in relation to their body size of any species of bird in the world...

s are a notable exception; their nares are located at the tip of their bills. A handful of species have no external nares. Cormorant
Cormorant
The bird family Phalacrocoracidae is represented by some 40 species of cormorants and shags. Several different classifications of the family have been proposed recently, and the number of genera is disputed.- Names :...

s and darter
Darter
The darters or snakebirds are mainly tropical waterbirds in the family Anhingidae. There are four living species, three of which are very common and widespread while the fourth is rarer and classified as near-threatened by the IUCN. The term "snakebird" is usually used without any additions to...

s have primitive external nares as nestlings, but these close soon after the birds fledge; adults of these species (and gannet
Gannet
Gannets are seabirds comprising the genus Morus, in the family Sulidae, closely related to the boobies.The gannets are large black and white birds with yellow heads. They have long pointed wings and long bills. Northern gannets are the largest seabirds in the North Atlantic, with a wingspan of up...

s and boobies
Booby
A booby is a seabird in the genus Sula, part of the Sulidae family. Boobies are closely related to the gannets , which were formerly included in Sula.-Description:...

 of all ages, which also lack external nostrils) breathe through their mouths. There is typically a septum
Nasal septum
The nasal septum separates the left and right airways in the nose, dividing the two nostrils.It is depressed by the Depressor septi nasi muscle.-Composition:The fleshy external end of the nasal septum is sometimes also called columella....

 made of bone or cartilage that separates the two nares, but in some families (including gulls, cranes and New World vultures) the septum is missing. 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. The feathers over a ptarmigan's nostrils help to warm the air it inhales, 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. These species, which include the albatrosses, petrels, diving petrels, storm petrels, fulmars and shearwaters, are widely known as "tubenoses". A number of species, including the falcon
Falcon
A falcon is any species of raptor in the genus Falco. The genus contains 37 species, widely distributed throughout Europe, Asia, and North America....

s, 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.

Operculum



The nares of some birds are covered by an operculum (plural opercula), a membraneous, horny or cartilaginous
Cartilage
Cartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...

 flap. In diving birds, the operculum keeps water out of the nasal cavity; when the birds dive, the impact force of the water closes the operculum. Some species which feed on flowers have opercula to help to keep pollen from clogging their nasal passages, while the opercula of the two species of Attagis seedsnipe
Attagis
Attagis is a genus of seedsnipe, a South American family of small gregarious waders which have adapted to a vegetarian diet.These birds look superficially like partridges in structure and bill shape. They have short legs and long wings...

 help to keep dust out. The nares of nestling Tawny Frogmouth
Tawny Frogmouth
The Tawny Frogmouth is an Australian species of frogmouth, a type of bird found throughout the Australian mainland, Tasmania and southern New Guinea. The Tawny Frogmouth is often mistaken to be an owl...

s 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. These 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; though it is sometimes referred to as the "cere", the latter is a different structure. Tapaculo
Tapaculo
The tapaculos are a group of small suboscine passeriform birds with numerous species, found mainly in South America and with the highest diversity in the Andean regions...

s are the only birds able to move their operculum.

Cere


Birds from a handful of families—including raptors, owls, skuas, parrots, turkeys and curassows—have a waxy structure called a cere (from the Latin cera, which means "wax") 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. In raptors, the cere is a sexual signal which indicates the "quality" of a bird; the orangeness of a Montague's Harrier's cere, for example, correlates to its body mass and physical condition. The cere color of young Eurasian Scops-Owls has an ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

 (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
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...

 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
Great Curassow
The Great Curassow is a large, pheasant-like bird from the Neotropics. At in length and in weight, this is a very large cracid. No other cracid match its maximum weight, but its length is matched by a few other cracids....

 has a yellow cere, which the female (and young males) lack. The male Budgerigar
Budgerigar
The Budgerigar , also known as Common Pet Parakeet or Shell Parakeet informally nicknamed the budgie, is a small, long-tailed, seed-eating parrot, and the only species in the Australian genus Melopsittacus...

's cere is blue, while the female's is pinkish or brown.

Nail



All wildfowl (ducks, geese, and swans) have a nail, a plate of hard horny tissue at the tip of the beak. This shield-shaped
Shield
A shield is a type of personal armor, meant to intercept attacks, either by stopping projectiles such as arrows or redirecting a hit from a sword, mace or battle axe to the side of the shield-bearer....

 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 duck
Diving duck
The diving ducks, commonly called pochards or scaups, are a category of duck which feed by diving beneath the surface of the water. They are part of the diverse and very large Anatidae family that includes ducks, geese, and swans....

s use theirs to pry molluscs from rocks. There is evidence that the nail may help a bird to grasp things; species which use strong grasping motions to secure their food have very wide nails. Certain types of mechanoreceptor
Mechanoreceptor
A mechanoreceptor is a sensory receptor that responds to mechanical pressure or distortion. There are four main types in the glabrous skin of humans: Pacinian corpuscles, Meissner's corpuscles, Merkel's discs, and Ruffini corpuscles...

s, 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
Greater Scaup
The Greater Scaup , just Scaup in Europe, or colloquially known as "Bluebill", for its bright blue bill, is small compared to other diving ducks, however it is larger than the closely related Lesser Scaup...

 has a wider black nail than does the very similar Lesser Scaup
Lesser Scaup
The Lesser Scaup is a small North American diving duck that migrates south as far as Central America in winter. It is colloquially known as the Little Bluebill or Broadbill because of its distinctive blue bill...

. 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".

Rictal bristles


Rictal bristles are stiff hair-like feathers that arise around the base of the 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. They may function as a "net", helping in the capture of flying prey, although to date, there has been no empirical evidence to support this idea. 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. They may also help to protect the eyes from particles encountered in flight, or from casual contact from vegetation. There is also evidence that the rictal bristles of some species may function tactilely, in a manner similar to that of mammal whiskers (vibrissae
Vibrissae
Vibrissae , or whiskers, are specialized hairs usually employed for tactile sensation. The term may also refer to the thick hairs found inside human nostrils, but these have no sensorial function and only operate as an airborne particulate barrier...

). Studies have shown that Herbst corpuscles, mechanoreceptors sensitive to pressure and vibration, are found in association with rictal bristles. They may help with prey detection, with navigation in darkened nest cavities, with the gathering of information during flight or with prey handling.

Egg tooth


Full-term chicks of most bird species have a small, sharp, calcified projection on their beak which they use to chip their way out of their egg
Egg (biology)
An egg is an organic vessel in which an embryo first begins to develop. In most birds, reptiles, insects, molluscs, fish, and monotremes, an egg is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside the body until the developing...

. Commonly known as an egg tooth, this white spike is generally located near the tip of the upper mandible, though some species have one near the tip of their lower mandible instead, and a few species have one on each mandible. Despite its name, the projection is not an actual tooth
Tooth
Teeth are small, calcified, whitish structures found in the jaws of many vertebrates that are used to break down food. Some animals, particularly carnivores, also use teeth for hunting or for defensive purposes. The roots of teeth are embedded in the Mandible bone or the Maxillary bone and are...

, as the similarly-named projections of some reptile
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...

s are; instead, it is part of the integumentary system
Integumentary system
The integumentary system is the organ system that protects the body from damage, comprising the skin and its appendages...

, as are claw
Claw
A claw is a curved, pointed appendage, found at the end of a toe or finger in most mammals, birds, and some reptiles. However, the word "claw" is also often used in reference to an invertebrate. Somewhat similar fine hooked structures are found in arthropods such as beetles and spiders, at the end...

s and scales
Scale (zoology)
In most biological nomenclature, a scale is a small rigid plate that grows out of an animal's skin to provide protection. In lepidopteran species, scales are plates on the surface of the insect wing, and provide coloration...

 are. 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. Once it has breached the egg's surface, the chick continues to chip at it until it has made a large hole. The weakened egg eventually shatters under the pressure of the bird's movements. The egg tooth is so critical to a successful escape from the egg that chicks of most species will perish unhatched if they fail to develop one. However, there are a few species which do not have egg teeth. Megapode chicks have an egg tooth while still in the egg but lose it before hatching, while kiwi
Kiwi
Kiwi are flightless birds endemic to New Zealand, in the genus Apteryx and family Apterygidae.At around the size of a domestic chicken, kiwi are by far the smallest living ratites and lay the largest egg in relation to their body size of any species of bird in the world...

 chicks never develop one; chicks of both families escape their eggs by kicking their way out. Most chicks lose their egg teeth within a few days of hatching, though petrel
Petrel
Petrels are tube-nosed seabirds in the bird order Procellariiformes. The common name does not indicate relationship beyond that point, as "petrels" occur in three of the four families within that group...

s keep theirs for nearly three weeks and Marbled Murrelet
Marbled Murrelet
The Marbled Murrelet is a small seabird from the North Pacific. It is a member of the auk family. It nests in old-growth forests or on the ground at higher latitudes where trees cannot grow...

s have theirs for up to a month. Generally, the egg tooth drops off, though in songbird
Songbird
A songbird is a bird belonging to the suborder Passeri of the perching birds . Another name that is sometimes seen as scientific or vernacular name is Oscines, from Latin oscen, "a songbird"...

s it is reabsorbed.

Color


The color of a bird's beak results from concentrations of pigment
Pigment
A pigment is a material that changes the color of reflected or transmitted light as the result of wavelength-selective absorption. This physical process differs from fluorescence, phosphorescence, and other forms of luminescence, in which a material emits light.Many materials selectively absorb...

s—primarily melanin
Melanin
Melanin is a pigment that is ubiquitous in nature, being found in most organisms . In animals melanin pigments are derivatives of the amino acid tyrosine. The most common form of biological melanin is eumelanin, a brown-black polymer of dihydroxyindole carboxylic acids, and their reduced forms...

s and carotenoids—in the epidermal layers, including the rhamphotheca. 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. Although it is thought to occur in combination with eumelanin in beaks which are buff, tan, or horn-colored, researchers have yet to isolate phaeomelanin from any beak structure. More than a dozen types of carotenoids are responsible for the coloration of most red, orange, and yellow beaks. The hue
Hue
Hue is one of the main properties of a color, defined technically , as "the degree to which a stimulus can be describedas similar to or different from stimuli that are described as red, green, blue, and yellow,"...

 of the color is determined by the precise mix of red and yellow pigments, while the saturation
Saturation (color theory)
In colorimetry and color theory, colorfulness, chroma, and saturation are related but distinct concepts referring to the perceived intensity of a specific color. Colorfulness is the degree of difference between a color and gray. Chroma is the colorfulness relative to the brightness of another color...

 is determined by the density of the deposited pigments. For example, bright red is created by dense deposits of mostly red pigments, while dull yellow is created by diffuse deposits of mostly yellow pigments. Bright orange is created by dense deposits of both red and yellow pigments, in roughly equal concentrations. Beak coloration helps to make displays using those beaks more obvious.

In general, beak color depends on a combination of the bird's hormonal state
Hormone
A hormone is a chemical released by a cell or a gland in one part of the body that sends out messages that affect cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. In essence, it is a chemical messenger that transports a signal from one...

 and diet
Diet (nutrition)
In nutrition, diet is the sum of food consumed by a person or other organism. Dietary habits are the habitual decisions an individual or culture makes when choosing what foods to eat. With the word diet, it is often implied the use of specific intake of nutrition for health or weight-management...

. Colors are typically brightest as the breeding season approaches, and palest after breeding.

Dimorphism



The size and shape of the beak can vary across species as well as between them; in some species, the size and proportions of the beak vary between males and females. This allows the sexes to utilize different ecological niches, thereby reducing intraspecific competition
Intraspecific competition
Intraspecific competition is a particular form of competition in which members of the same species vie for the same resource in an ecosystem...

. For example, females of nearly all shorebirds have longer bills than males of the same species, and female American Avocet
American Avocet
The American Avocet is a large wader in the avocet and stilt family, Recurvirostridae.This avocet has long, thin, gray legs, giving it its colloquial name, "blue shanks". The plumage is black and white on the back with white on the underbelly. The neck and head are cinnamon colored in the summer...

s 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 hornbill
Hornbill
Hornbills are a family of bird found in tropical and subtropical Africa, Asia and Melanesia. They are characterized by a long, down-curved bill which is frequently brightly-colored and sometimes has a casque on the upper mandible. Both the common English and the scientific name of the family...

s show sexual dimorphism
Sexual dimorphism
Sexual dimorphism is a phenotypic difference between males and females of the same species. Examples of such differences include differences in morphology, ornamentation, and behavior.-Examples:-Ornamentation / coloration:...

 in the size and shape of both beaks and casque
Casque
Casque can refer to:* French word for helmet* An enlargement on the upper mandible of the bill of some species of birds, including many hornbills*Hornbill ivory: the casque of the Helmeted Hornbill, collected as a decorative material...

s, and the female Huia
Huia
The Huia was the largest species of New Zealand wattlebird and was endemic to the North Island of New Zealand. Its extinction in the early 20th century had two primary causes. The first was rampant overhunting to procure Huia skins for mounted specimens, which were in worldwide demand by...

's slim, decurved bill was nearly twice as long as the male's straight, thicker one.

Color can also differ between sexes or ages within a species. Typically, such a color difference is due to the presence of androgen
Androgen
Androgen, also called androgenic hormone or testoid, is the generic term for any natural or synthetic compound, usually a steroid hormone, that stimulates or controls the development and maintenance of male characteristics in vertebrates by binding to androgen receptors...

s. For example, in House Sparrow
House Sparrow
The House Sparrow is a bird of the sparrow family Passeridae, found in most parts of the world. One of about 25 species in the genus Passer, the House Sparrow occurs naturally in most of Europe, the Mediterranean region, and much of Asia...

s, melanins are produced only in the presence of testosterone
Testosterone
Testosterone is a steroid hormone from the androgen group and is found in mammals, reptiles, birds, and other vertebrates. In mammals, testosterone is primarily secreted in the testes of males and the ovaries of females, although small amounts are also secreted by the adrenal glands...

; castrated
Castration
Castration is any action, surgical, chemical, or otherwise, by which a male loses the functions of the testicles or a female loses the functions of the ovaries.-Humans:...

 male House Sparrows—like female House Sparrows—have brown beaks. Castration also prevents the normal seasonal color change in the beaks of male Black-headed Gull
Black-headed Gull
The Black-headed Gull is a small gull which breeds in much of Europe and Asia, and also in coastal eastern Canada. Most of the population is migratory, wintering further south, but some birds in the milder westernmost areas of Europe are resident...

s and Indigo Bunting
Indigo Bunting
The Indigo Bunting, Passerina cyanea, is a small seed-eating bird in the family Cardinalidae. It is migratory, ranging from southern Canada to northern Florida during the breeding season, and from southern Florida to northern South America during the winter. It often migrates by night, using the...

s.

Functions



Birds may bite or stab with their beaks to defend themselves. Some species use their beaks in displays of various sorts. As part of his courtship, for example, the male Garganey
Garganey
The Garganey is a small dabbling duck. It breeds in much of Europe and western Asia, but is strictly migratory, with the entire population moving to southern Africa, India Santragachi and Australasia in winter, where large flocks can occur. This species was first described by Linnaeus in 1758...

 touches his beak to the blue speculum feathers on his wings in a fake preening display, and the male Mandarin Duck
Mandarin Duck
The Mandarin Duck , or just Mandarin, is a medium-sized perching duck, closely related to the North American Wood Duck. It is 41–49 cm long with a 65–75 cm wingspan.-Description:...

 does the same with his orange sail feathers. A number of species use a gaping, open beak in their fear and/or threat displays. Some augment the display by hissing or breathing heavily, while others clap their beaks.

Communication


A number of species, including stork
Stork
Storks are large, long-legged, long-necked wading birds with long, stout bills. They belong to the family Ciconiidae. They are the only family in the biological order Ciconiiformes, which was once much larger and held a number of families....

s, some owl
Owl
Owls are a group of birds that belong to the order Strigiformes, constituting 200 bird of prey species. Most are solitary and nocturnal, with some exceptions . Owls hunt mostly small mammals, insects, and other birds, although a few species specialize in hunting fish...

s, frogmouth
Frogmouth
The frogmouths are a group of nocturnal birds related to the nightjars. They are found from India across southern Asia to Australia.They are named for their large flattened hooked bills and huge frog-like gape, which they use to capture insects. Their flight is weak.They rest horizontally on...

s and the Noisy Miner
Noisy Miner
The Noisy Miner is a bird common to the eastern and southern states of Australia. It ranges from northern Queensland along the eastern coast to South Australia and Tasmania. Its typical diet consists of nectar, fruit and insects, and occasionally it feeds on small reptiles or amphibians...

, use bill clapping as a form of communication.

Heat exchange


Studies have shown that some birds use their beaks to rid themselves of excess heat. The Toco Toucan
Toco Toucan
The Toco Toucan is the largest and probably the best known species in the toucan family. It is found in semi-open habitats throughout a large part of central and eastern South America...

, which has the largest beak relative to the size of its body of any bird species, is capable of modifying the blood flow
Blood flow
Blood flow is the continuous running of blood in the cardiovascular system.The human body is made up of several processes all carrying out various functions. We have the gastrointestinal system which aids the digestion and the absorption of food...

 to its beak. This process allows the beak to work as a "transient thermal radiator", reportedly rivaling an elephant's ears in its ability to radiate body heat. Measurements of the bill sizes of several species of American sparrow
American sparrow
American sparrows are a group of mainly New World passerine birds, forming part of the family Emberizidae. American sparrows are seed-eating birds with conical bills, brown or gray in color, and many species have distinctive head patterns....

s found in salt marsh
Salt marsh
A salt marsh is an environment in the upper coastal intertidal zone between land and salt water or brackish water, it is dominated by dense stands of halophytic plants such as herbs, grasses, or low shrubs. These plants are terrestrial in origin and are essential to the stability of the salt marsh...

es along the North American coastlines show a strong correlation with summer temperatures recorded in the locations where the sparrows breed; latitude
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...

 alone showed a much weaker correlation. 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. Several ratite
Ratite
A ratite is any of a diverse group of large, flightless birds of Gondwanan origin, most of them now extinct. Unlike other flightless birds, the ratites have no keel on their sternum—hence the name from the Latin ratis...

s, including the Common Ostrich, the Emu
Emu
The Emu Dromaius novaehollandiae) is the largest bird native to Australia and the only extant member of the genus Dromaius. It is the second-largest extant bird in the world by height, after its ratite relative, the ostrich. There are three subspecies of Emus in Australia...

 and the Southern Cassowary
Southern Cassowary
The Southern Cassowary, Casuarius casuarius, also known as Double-wattled Cassowary, Australian Cassowary or Two-wattled Cassowary, is a large flightless black bird...

, use various bare parts of their bodies (including their beaks) to dissipate as much as 40% of their metabolic heat production.

Billing



During courtship, mated pairs of many bird species touch or clasp each other's bills. Termed billing (also nebbing in British English), this behavior appears to strengthen pair bond
Pair bond
In biology, a pair bond is the strong affinity that develops in some species between the males and females in a pair, potentially leading to breeding. Pair-bonding is a term coined in the 1940s that is frequently used in sociobiology and evolutionary psychology circles...

ing. The amount of contact involved varies among species. Some gently touch only a part of their partner's beak while others clash their beaks vigorously together.

Gannet
Gannet
Gannets are seabirds comprising the genus Morus, in the family Sulidae, closely related to the boobies.The gannets are large black and white birds with yellow heads. They have long pointed wings and long bills. Northern gannets are the largest seabirds in the North Atlantic, with a wingspan of up...

s raise their bills high and repeatedly clatter them, the male puffin
Puffin
Puffins are any of three small species of auk in the bird genus Fratercula with a brightly coloured beak during the breeding season. These are pelagic seabirds that feed primarily by diving in the water. They breed in large colonies on coastal cliffs or offshore islands, nesting in crevices among...

 nibbles at the female's beak, the male waxwing
Waxwing
The waxwings form the genus Bombycilla of passerine birds. According to most authorities, this is the only genus placed in the family Bombycillidae.-Description:Waxwings are characterised by soft silky plumage...

 puts his bill in the female's mouth and raven
Raven
Raven is the common name given to several larger-bodied members of the genus Corvus—but in Europe and North America the Common Raven is normally implied...

s hold each other's beaks in a prolonged "kiss". Billing can also be used as a gesture of appeasement or subordination. Subordinate Gray Jay
Gray Jay
The Gray Jay , also Grey Jay, Canada Jay, or Whiskey Jack, is a member of the crow and jay family found in the boreal forests across North America north to the tree-line and in subalpine forests of the Rocky Mountains south to New Mexico and Arizona...

 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. A number of parasites, including rhinonyssids
Rhinonyssidae
Rhinonyssidae is a family of mites in the order Mesostigmata.-Species:Agapornyssus Gretillat, Capron & Brygoo, 1959*Agapornyssus faini Gretillat, Capron & Brygoo, 1959Astridiella Fain, 1957*Astridiella scotornis...

 and Trichomonas gallinae
Trichomonas gallinae
The protozoan Trichomonas gallinae is a cosmopolitan parasite of pigeons and doves. Other birds such as domestic and wild turkeys, chickens, raptors may also become infected. The disease in pigeons is commonly called "canker"...

are known to be transferred between birds during episodes of billing.

Usage of the term has spread beyond avian behavior; "billing and cooing" in reference to human courtship (particularly kissing) has been in use since Shakespeare's
William Shakespeare
William Shakespeare was an English poet and playwright, widely regarded as the greatest writer in the English language and the world's pre-eminent dramatist. He is often called England's national poet and the "Bard of Avon"...

 time, and derives from the courtship of doves.

Beak trimming



Because the beak is a sensitive organ with many sensory receptors, beak trimming is "acutely painful" to the birds it is performed on. It is nonetheless routinely done to intensively farmed poultry
Poultry
Poultry are domesticated birds kept by humans for the purpose of producing eggs, meat, and/or feathers. These most typically are members of the superorder Galloanserae , especially the order Galliformes and the family Anatidae , commonly known as "waterfowl"...

 flocks, particularly laying and broiler
Broiler
A broiler is a type of chicken raised specifically for meat production. Modern commercial broilers, typically known as Cornish crosses or Cornish-Rocks are specially bred for large scale, efficient meat production and grow much faster than egg or traditional dual purpose breeds...

 breeder flocks, because it helps to reduce a number of stress
Stress (biology)
Stress is a term in psychology and biology, borrowed from physics and engineering and first used in the biological context in the 1930s, which has in more recent decades become commonly used in popular parlance...

-induced behaviors, including cannibalism
Cannibalism (zoology)
In zoology, cannibalism is the act of one individual of a species consuming all or part of another individual of the same species as food. Cannibalism is a common ecological interaction in the animal kingdom and has been recorded for more than 1500 species...

, vent pecking
Vent pecking
Vent pecking is an abnormal behaviour of birds performed primarily by commercial egg-laying hens. It is characterised by pecking damage to the cloaca, the surrounding skin and underlying tissue...

 and feather pecking
Feather pecking
Feather pecking is a behavioural problem that occurs most frequently amongst domestic hens reared for egg production, although it does occur in other poultry such as pheasants, turkeys, ducks and is sometimes seen in farmed ostriches. Feather pecking occurs when one bird repeatedly pecks at the...

. A cauterizing blade or infrared beam is used to cut off about half of the upper beak and about a third of the lower beak. Pain and sensitivity can persist for weeks or months after the procedure, and neuromas can form along the cut edges. Food intake typically decreases for some period after the beak is trimmed. However, studies show that trimmed poultry's adrenal gland
Adrenal gland
In mammals, the adrenal glands are endocrine glands that sit atop the kidneys; in humans, the right suprarenal gland is triangular shaped, while the left suprarenal gland is semilunar shaped...

s weigh less, and their plasma corticosterone
Corticosterone
Corticosterone is a 21-carbon steroid hormone of the corticosteroid type produced in the cortex of the adrenal glands.-Roles:In many species, including amphibians, reptiles, rodents and birds, corticosterone is a main glucocorticoid, involved in regulation of fuel, immune reactions, and stress...

levels are lower than those found in untrimmed poultry, indicating that they are less stressed overall.