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The beak, bill, or rostrum is an external anatomical structure found mostly in
birds Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweigh ...
, but also in
turtles Turtles are an order (biology), order of reptiles known as Testudines, characterized by a special turtle shell, shell developed mainly from their ribs. Modern turtles are divided into two major groups, the Pleurodira (side necked turtles) an ...
,
non-avian dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
s and a few mammals. A beak is used for eating,
preening Preening is a found in birds that involves the use of the beak to position feathers, interlock feather that have become separated, clean plumage, and keep ectoparasites in check. Feathers contribute significantly to a bird's insulation, waterp ...
, manipulating objects, killing prey, fighting, probing for food,
courtship Courtship is the period wherein some couples get to know each other prior to a possible marriage. Courtship traditionally may begin after a betrothal and may conclude with the celebration of marriage. A courtship may be an informal and private m ...
, and feeding young. The terms ''beak'' and ''
rostrum Rostrum may refer to: * Any kind of a platform for a speaker: **dais **pulpit * Rostrum (anatomy), a beak, or anatomical structure resembling a beak, as in the mouthparts of many sucking insects * Rostrum (ship), a form of bow on naval ships * Ros ...
'' are also used to refer to a similar mouth part in some
ornithischia Ornithischia () is an extinct order of mainly herbivorous dinosaurs characterized by a pelvic structure superficially similar to that of birds. The name ''Ornithischia'', or "bird-hipped", reflects this similarity and is derived from the Greek s ...
ns,
pterosaur Pterosaurs (; from Greek ''pteron'' and ''sauros'', meaning "wing lizard") is an extinct clade of flying reptiles in the order, Pterosauria. They existed during most of the Mesozoic: from the Late Triassic to the end of the Cretaceous (228 to ...
s,
cetacea Cetacea (; , ) is an infraorder of aquatic mammals that includes whales, dolphins, and porpoises. Key characteristics are their fully aquatic lifestyle, streamlined body shape, often large size and exclusively carnivorous diet. They propel them ...
ns,
dicynodont Dicynodontia is an extinct clade of anomodonts, an extinct type of non-mammalian therapsid. Dicynodonts were herbivorous animals with a pair of tusks, hence their name, which means 'two dog tooth'. Members of the group possessed a horny, typicall ...
s,
anuran A frog is any member of a diverse and largely carnivorous group of short-bodied, tailless amphibians composing the order Anura (ανοὐρά, literally ''without tail'' in Ancient Greek). The oldest fossil "proto-frog" ''Triadobatrachus'' ...
tadpole A tadpole is the larval stage in the biological life cycle of an amphibian. Most tadpoles are fully aquatic, though some species of amphibians have tadpoles that are terrestrial. Tadpoles have some fish-like features that may not be found i ...
s,
monotreme Monotremes () are prototherian mammals of the order Monotremata. They are one of the three groups of living mammals, along with placentals (Eutheria), and marsupials (Metatheria). Monotremes are typified by structural differences in their brain ...
s (i.e.
echidna Echidnas (), sometimes known as spiny anteaters, are quill-covered monotremes (egg-laying mammals) belonging to the family Tachyglossidae . The four extant species of echidnas and the platypus are the only living mammals that lay eggs and the ...
s and
platypus The platypus (''Ornithorhynchus anatinus''), sometimes referred to as the duck-billed platypus, is a semiaquatic, egg-laying mammal Endemic (ecology), endemic to Eastern states of Australia, eastern Australia, including Tasmania. The platypu ...
es, which have a beak-like structure),
siren Siren or sirens may refer to: Common meanings * Siren (alarm), a loud acoustic alarm used to alert people to emergencies * Siren (mythology), an enchanting but dangerous monster in Greek mythology Places * Siren (town), Wisconsin * Siren, Wisc ...
s,
pufferfish Tetraodontidae is a family of primarily marine and estuarine fish of the order Tetraodontiformes. The family includes many familiar species variously called pufferfish, puffers, balloonfish, blowfish, blowies, bubblefish, globefish, swellfis ...
,
billfish The term billfish refers to a group of saltwater predatory fish characterised by prominent pointed bills (rostra), and by their large size; some are longer than . Extant billfish include sailfish and marlin, which make up the family Istiophor ...
es and
cephalopods A cephalopod is any member of the molluscan class Cephalopoda (Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, an ...
. 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 Birds of prey or predatory birds, also known as raptors, are hypercarnivorous bird species that actively hunt and feed on other vertebrates (mainly mammals, reptiles and other smaller birds). In addition to speed and strength, these predators ...
, in modern
ornithology Ornithology is a branch of zoology that concerns the "methodological study and consequent knowledge of birds with all that relates to them." Several aspects of ornithology differ from related disciplines, due partly to the high visibility and th ...
, the terms beak and bill are generally considered to be synonymous. The word, which dates from the 13th century, comes from the
Middle English Middle English (abbreviated to ME) is a form of the English language that was spoken after the Norman conquest of 1066, until the late 15th century. The English language underwent distinct variations and developments following the Old English p ...
''bec'', which itself comes from the Latin ''beccus''.


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). The upper, and in some cases the lower, mandibles are strengthened internally by a complex
three-dimensional Three-dimensional space (also: 3D space, 3-space or, rarely, tri-dimensional space) is a geometric setting in which three values (called ''parameters'') are required to determine the position of an element (i.e., point). This is the informal ...
network of bony spicules (or
trabecula A trabecula (plural trabeculae, from Latin for "small beam") is a small, often microscopic, tissue element in the form of a small beam, strut or rod that supports or anchors a framework of parts within a body or organ. A trabecula generally has ...
e) seated in soft
connective tissue Connective tissue is one of the four primary types of animal tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesenchyme derived from the mesoderm the middle embryonic germ layer. Connective tiss ...
and surrounded by the hard outer layers of the beak. The avian jaw apparatus is made up of two units: one
four-bar linkage In the study of mechanisms, a four-bar linkage, also called a four-bar, is the simplest closed- chain movable linkage. It consists of four bodies, called ''bars'' or ''links'', connected in a loop by four joints. Generally, the joints are configu ...
mechanism and one five-bar linkage mechanism.


Mandibles

The upper mandible is supported by a three-pronged
bone A bone is a Stiffness, rigid Organ (biology), organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red blood cell, red and white blood cells, store minerals, provid ...
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 The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, the ...
. 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
ratite A ratite () is any of a diverse group of flightless, large, long-necked, and long-legged birds of the infraclass Palaeognathae. Kiwi, the exception, are much smaller and shorter-legged and are the only nocturnal extant ratites. The systematics ...
s. Here, the
vomer The vomer (; lat, vomer, lit=ploughshare) 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 maxill ...
is 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. 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 joint 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 The huia ( ; ; ''Heteralocha acutirostris'') is an extinct species of New Zealand wattlebird, endemic to the North Island of New Zealand. The last confirmed sighting of a huia was in 1907, although there was a credible sighting in 1924. It ...
, which have well-developed
digastric muscle The digastric muscle (also digastricus) (named ''digastric'' as it has two 'bellies') is a small muscle located under the jaw. The term "digastric muscle" refers to this specific muscle. However, other muscles that have two separate muscle bellie ...
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 of a thin sheath of
keratin Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. Alpha-keratin (α-keratin) is a type of keratin found in vertebrates. It is the key structural material making up scales, hair, nails, feathers, ho ...
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 layer The Malpighian layer (''stratum mucosum'' or ''stratum malpighii'') of the epidermis, the outermost layer of the skin, is generally defined as both the stratum basale (basal layer) and the thicker stratum spinosum (spinous layer/prickle cell laye ...
of the bird's
epidermis The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the amount of water rele ...
, growing from plates at the base of each mandible. There is a
vascular The blood vessels are the components of the circulatory system that transport blood throughout the human body. These vessels transport blood cells, nutrients, and oxygen to the tissues of the body. They also take waste and carbon dioxide away f ...
layer between the rhamphotheca and the deeper layers of the
dermis The dermis or corium is a layer of skin between the epidermis (with which it makes up the cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from stress and strain. It is divided i ...
, which is attached directly to the
periosteum The periosteum is a membrane that covers the outer surface of all bones, except at the articular surfaces (i.e. the parts within a joint space) of long bones. Endosteum lines the inner surface of the medullary cavity of all long bones. Structure ...
of the bones of the beak.Samour (2000), p. 296. The rhamphotheca grows continuously in most birds, and in some species, the color varies seasonally. In some
alcids An auk or alcid is a bird of the family Alcidae in the order Charadriiformes. The alcid family includes the murres, guillemots, auklets, puffins, and murrelets. The word "auk" is derived from Icelandic ''álka'', from Old Norse ''alka'' (a ...
, 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 Extant is the opposite of the word extinct. It may refer to: * Extant hereditary titles * Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English * Extant taxon, a taxon which is not extinct, ...
birds have a single seamless rhamphotheca, species in a few families, including the
albatross Albatrosses, of the biological family Diomedeidae, are large seabirds related to the procellariids, storm petrels, and diving petrels in the order Procellariiformes (the tubenoses). They range widely in the Southern Ocean and the North Pacifi ...
es and the
emu The emu () (''Dromaius novaehollandiae'') is the second-tallest living bird after its ratite relative the ostrich. It is endemic to Australia where it is the largest native bird and the only extant member of the genus ''Dromaius''. The emu' ...
, 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.


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 that allow them to handle their typical food sources better.
Granivorous Seed predation, often referred to as granivory, is a type of plant-animal interaction in which granivores (seed predators) feed on the seeds of plants as a main or exclusive food source,Hulme, P.E. and Benkman, C.W. (2002) "Granivory", pp. 132 ...
(seed-eating) birds, for example, have ridges in their tomia, which help the bird to slice through a seed's outer hull. Most
falcon Falcons () are birds of prey in the genus ''Falco'', which includes about 40 species. Falcons are widely distributed on all continents of the world except Antarctica, though closely related raptors did occur there in the Eocene. Adult falcons ...
s 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 A kite is a tethered heavier than air flight, heavier-than-air or lighter-than-air craft with wing surfaces that react against the air to create Lift (force), lift and Drag (physics), drag forces. A kite consists of wings, tethers and anchors. ...
, principally those that prey on insects or lizards, also have one or more of these sharp projections, as do the
shrike Shrikes () are passerine birds of the family Laniidae. The family is composed of 34 species in four genera. The family name, and that of the largest genus, ''Lanius'', is derived from the Latin word for "butcher", and some shrikes are also know ...
s. The tomial teeth of falcons are underlain by bone, while the shrike tomial teeth are entirely keratinous. Some fish-eating species, e.g., the
merganser ''Mergus'' is the genus of the typical mergansers , fish-eating ducks in the subfamily Anatinae. The genus name is a Latin word used by Pliny the Elder and other Roman authors to refer to an unspecified waterbird. The common merganser (''Merg ...
s, 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
insectivores A robber fly eating a hoverfly An insectivore is a carnivorous animal or plant that eats insects. An alternative term is entomophage, which can also refer to the human practice of eating insects. The first vertebrate insectivores wer ...
(preferring hard-shelled prey) or
snail A snail is, in loose terms, a shelled gastropod. The name is most often applied to land snails, terrestrial pulmonate gastropod molluscs. However, the common name ''snail'' is also used for most of the members of the molluscan class Gastro ...
eaters, and the brush-like projections may help to increase the
coefficient of friction Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction: *Dry friction is a force that opposes the relative lateral motion of ...
between the mandibles, thereby improving the bird's ability to hold hard prey items. Serrations on
hummingbird Hummingbirds are birds native to the Americas and comprise the biological family Trochilidae. With about 361 species and 113 genera, they occur from Alaska to Tierra del Fuego, but the vast majority of the species are found in the tropics aro ...
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 The snow goose (''Anser caerulescens'') is a species of goose native to North America. Both white and dark morphs exist, the latter often known as blue goose. Its name derives from the typically white plumage. The species was previously placed ...
, for example, has a reddish-pink bill with black tomia, while the whole beak of the similar
Ross's goose The Ross's goose (''Anser rossii'') is a white goose with black wingtips and a relatively short neck. It is the smallest of the three " white geese" that breed in North America. It is similar in appearance to a white-phase snow goose, but about 4 ...
is pinkish-red, without darker tomia.


Culmen

The culmen is the
dorsal Dorsal (from Latin ''dorsum'' ‘back’) may refer to: * Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism * Dorsal, positioned on top of an aircraft's fuselage * Dorsal co ...
ridge of the upper mandible. Likened by ornithologist E. Coues 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) 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 (measured in a straight line from point to point, ignoring any curve in the culmen) taken with
calipers A caliper (British spelling also calliper, or in plurale tantum sense a pair of calipers) is a device used to measure the dimensions of an object. Many types of calipers permit reading out a measurement on a ruled scale, a dial, or a digital dis ...
. 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 The parrot crossbill (''Loxia pytyopsittacus'') is a small passerine bird in the finch family Fringillidae. It breeds in pine forests in northern and northeastern Europe. Taxonomy The parrot crossbill was formerly described in 1793 by the German ...
is strongly decurved, while that of the very similar
red crossbill The red crossbill or common crossbill (''Loxia curvirostra'') is a small passerine bird in the finch family Fringillidae. Crossbills have distinctive mandibles, crossed at the tips, which enable them to extract seeds from conifer cones and other ...
is more moderately curved. The culmen of a juvenile
common loon The common loon or great northern diver (''Gavia immer'') is a large member of the loon, or diver, family of birds. Breeding adults have a plumage that includes a broad black head and neck with a greenish, purplish, or bluish sheen, blackish o ...
is all dark, while that of the very similarly
plumage Plumage ( "feather") is a layer of feathers that covers a bird and the pattern, colour, and arrangement of those feathers. The pattern and colours of plumage differ between species and subspecies and may vary with age classes. Within species, ...
d juvenile
yellow-billed loon The yellow-billed loon (''Gavia adamsii''), 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 pale towards the tip.


Gonys

The gonys is the
ventral Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek language, Greek roots, describe something in its standard anatomical position. Th ...
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.Howell (2007), p. 23. 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, or alternately, to the full-length apposition of the closed mandibles, from the corners of the mouth to the tip of the beak.


Gape

In
bird anatomy Bird anatomy, or the physiological structure of birds' bodies, shows many unique adaptations, mostly aiding flight. Birds have a light skeletal system and light but powerful musculature which, along with circulatory and respiratory systems capabl ...
, the gape is the interior of the open mouth of a bird, and the gape flange is the region where the two
mandible In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower tooth, teeth in place. The mandible sits beneath the maxilla. It is the only movabl ...
s join together at the base of the beak. The width of the gape can be a factor in the choice of food. Gapes of juvenile
altricial In biology, altricial species are those in which the young are underdeveloped at the time of birth, but with the aid of their parents mature after birth. Precocial species are those in which the young are relatively mature and mobile from the mome ...
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 this, the parents decide how to distribute food among the chicks in the nest. Some species, especially in the families
Viduidae __NOTOC__ The indigobirds and whydahs, together with the cuckoo-finch, make up the family Viduidae; they are small passerine birds native to Africa. These are finch-like species which usually have black or indigo predominating in their plumage. ...
and
Estrildidae Estrildidae, or estrildid finches, is a family of small seed-eating passerine birds of the Old World tropics and Australasia. They comprise species commonly known as munias, mannikins, firefinches, parrotfinches and waxbills. Despite the word "fi ...
, 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 A passerine () is any bird of the order Passeriformes (; from Latin 'sparrow' and '-shaped'), which includes more than half of all bird species. Sometimes known as perching birds, passerines are distinguished from other orders of birds by t ...
species found that the gapes were conspicuous in the
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...
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 The barn swallow (''Hirundo rustica'') is the most widespread species of swallow in the world. In fact, it appears to have the largest natural distribution of any of the world's passerines, ranging over 251 million square kilometres globally. ...
nestlings showed the vividness of the gape was positively
correlated In statistics, correlation or dependence is any statistical relationship, whether causal or not, between two random variables or bivariate data. Although in the broadest sense, "correlation" may indicate any type of association, in statistics ...
with
T-cell A T cell is a type of lymphocyte. T cells are one of the important white blood cells of the immune system and play a central role in the adaptive immune response. T cells can be distinguished from other lymphocytes by the presence of a T-cell rec ...
–mediated
immunocompetence In immunology, immunocompetence is the ability of the body to produce a normal immune response following exposure to an antigen. Immunocompetence is the opposite of immunodeficiency (also known as ''immuno-incompetence'' or being ''immuno-compro ...
, and that larger brood size and injection with an
antigen In immunology, an antigen (Ag) is a molecule or molecular structure or any foreign particulate matter or a pollen grain that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune response. ...
led to a less vivid gape. Conversely, the red gape of the
common cuckoo The common cuckoo (''Cuculus canorus'') is a member of the cuckoo order of birds, Cuculiformes, which includes the roadrunners, the anis and the coucals. This species is a widespread summer migrant to Europe and Asia, and winters in Africa. I ...
(''Cuculus canorus'') did not induce extra feeding in host parents. Some
brood parasite Brood parasites are animals that rely on others to raise their young. The strategy appears among birds, insects and fish. The brood parasite manipulates a host, either of the same or of another species, to raise its young as if it were its own ...
s, such as the
Hodgson's hawk-cuckoo Hodgson's hawk-cuckoo (''Hierococcyx nisicolor''), also known as the whistling hawk-cuckoo is a species of cuckoo found in north-eastern India, Myanmar, southern China China, officially the People's Republic of China (PRC), is a country ...
(''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

Most species of birds have external nares (
nostril A nostril (or naris , plural ''nares'' ) is either of the two orifices of the nose. They enable the entry and exit of air and other gasses through the nasal cavities. In birds and mammals, they contain branched bones or cartilages called turbi ...
s) located somewhere on their beak. The nares are two holes—circular, oval or slit-like in shape—which lead to the
nasal cavities The nasal cavity is a large, air-filled space above and behind the nose in the middle of the face. The nasal septum divides the cavity into two cavities, also known as fossae. Each cavity is the continuation of one of the two nostrils. The nasal c ...
within the bird's skull, and thus to the rest of the
respiratory system The respiratory system (also respiratory apparatus, ventilatory system) is a biological system consisting of specific organs and structures used for gas exchange in animals and plants. The anatomy and physiology that make this happen varies grea ...
. In most bird species, the nares are located in the basal third of the upper mandible.
Kiwi Kiwi most commonly refers to: * Kiwi (bird), a flightless bird native to New Zealand * Kiwi (nickname), a nickname for New Zealanders * Kiwifruit, an edible berry * Kiwi dollar or New Zealand dollar, a unit of currency Kiwi or KIWI may also refe ...
are a notable exception; their nares are located at the tip of their bills. A handful of species have no external nares.
Cormorant Phalacrocoracidae is a family of approximately 40 species of aquatic birds commonly known as cormorants and shags. Several different classifications of the family have been proposed, but in 2021 the IOC adopted a consensus taxonomy of seven ge ...
s and
darter The darters, anhingas, or snakebirds are mainly tropical waterbirds in the family Anhingidae, which contains a single genus, ''Anhinga''. There are four living species, three of which are very common and widespread while the fourth is rarer and ...
s have primitive external nares as nestlings, but these close soon after the birds
fledge Fledging is the stage in a flying animal's life between hatching or birth and becoming capable of flight. This term is most frequently applied to birds, but is also used for bats. For altricial birds, those that spend more time in vulnerable c ...
; adults of these species (and
gannet Gannets are seabirds comprising the genus ''Morus'' in the family Sulidae, closely related to boobies. Gannets are large white birds with yellowish heads; black-tipped wings; and long bills. Northern gannets are the largest seabirds in the Nor ...
s and
boobies A booby is a seabird in the genus ''Sula'', part of the family Sulidae. Boobies are closely related to the gannets (''Morus''), which were formerly included in ''Sula''. Systematics and evolution The genus ''Sula'' was introduced by the Frenc ...
of all ages, which also lack external nostrils) breathe through their mouths. There is typically a
septum In biology, a septum (Latin for ''something that encloses''; plural septa) is a wall, dividing a cavity or structure into smaller ones. A cavity or structure divided in this way may be referred to as septate. Examples Human anatomy * Interatri ...
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 Procellariiformes is an order (biology), order of seabirds that comprises four family (biology), families: the albatrosses, the Procellariidae, petrels and shearwaters, and two families of storm petrels. Formerly called Tubinares and still call ...
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 Falcons () are birds of prey in the genus ''Falco'', which includes about 40 species. Falcons are widely distributed on all continents of the world except Antarctica, though closely related raptors did occur there in the Eocene. Adult falcons ...
s, have a small bony
tubercule In anatomy, a tubercle (literally 'small tuber', Latin for 'lump') is any round Nodule (medicine), nodule, small wikt:eminence, eminence, or warty outgrowth found on external or internal Organ (anatomy), organs of a plant or an animal. In plant ...
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 is a resilient and smooth type of connective tissue. In tetrapods, it covers and protects the ends of long bones at the joints as articular cartilage, and is a structural component of many body parts including the rib cage, the neck and ...
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 help to keep dust out. The nares of nestling
tawny frogmouth The tawny frogmouth (''Podargus strigoides'') is a species of frogmouth native to the Australian mainland and Tasmania and found throughout. It is a big-headed, stocky bird, often mistaken for an owl, due to its nocturnal habits and similar colour ...
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
pigeon Columbidae () is a bird family consisting of doves and pigeons. It is the only family in the order Columbiformes. These are stout-bodied birds with short necks and short slender bills that in some species feature fleshy ceres. They primarily ...
s, 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 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 eating, preening, manipulating objects, killing prey, fighting, probing for food, ...
'', this is a different structure.
Tapaculo The tapaculos or tapacolos are a family, Rhinocryptidae, of small suboscine passerine birds, found mainly in South America and with the highest diversity in the Andean regions. Three species ( Chocó, Tacarcuna, and the silvery-fronted) are ...
s are the only birds known to have the ability to move their opercula.


Rosette

Some species, such as the
puffin Puffins are any of three species of small alcids (auks) in the bird genus ''Fratercula''. 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 crev ...
, have a fleshy rosette, sometimes called a "gape rosette", at the corners of the beak. In the puffin, this is grown as part of its display plumage.


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 Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...
''cera'', which means "wax") or ceroma which covers the base of their bill. This structure typically contains the nares, except in the owls, where the nares are
distal Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...
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
Montagu's harrier Montagu's harrier (''Circus pygargus'') is a migratory bird of prey of the harrier family. Its common name commemorates the British naturalist George Montagu. Taxonomy The first formal description of Montagu's harrier was by the Swedish na ...
's cere, for example, correlates to its body mass and physical condition. The cere color of young Eurasian scops-owls has an
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...
(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 In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
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 The budgerigar ( ; ''Melopsittacus undulatus''), also known as the common parakeet or shell parakeet, is a small, long-tailed, seed-eating parrot usually nicknamed the budgie ( ), or in American English, the parakeet. Budgies are the only spe ...
's cere is royal blue, while the female's is a very pale blue, white, or brown.


Nail

All birds of the family
Anatidae The Anatidae are the biological family of water birds that includes ducks, geese, and swans. The family has a cosmopolitan distribution, occurring on all the world's continents except Antarctica. These birds are adapted for swimming, floating ...
(ducks, geese, and swans) have a nail, a plate of hard horny tissue at the tip of the beak. This
shield A shield is a piece of personal armour held in the hand, which may or may not be strapped to the wrist or forearm. Shields are used to intercept specific attacks, whether from close-ranged weaponry or projectiles such as arrows, by means of a ...
-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 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 Anatidae, the diverse and very large family that includes ducks, geese, and swans. The diving d ...
s use theirs to pry
mollusc Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000  extant species of molluscs are recognized. The number of fossil species is esti ...
s 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
mechanoreceptor A mechanoreceptor, also called mechanoceptor, is a sensory receptor that responds to mechanical pressure or distortion. Mechanoreceptors are innervated by sensory neurons that convert mechanical pressure into electrical signals that, in animals, ...
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 The greater scaup (''Aythya marila''), just scaup in Europe or, colloquially, "bluebill" in North America, is a mid-sized diving duck, larger than the closely related lesser scaup. It spends the summer months breeding in Alaska, northern Canada, ...
has a wider black nail than does the very similar
lesser scaup The lesser scaup (''Aythya affinis'') 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. The origin of ...
. Juvenile "
grey geese The waterfowl genus ''Anser'' includes the grey geese and the white geese. It belongs to the true geese and swan subfamily (Anserinae). The genus has a Holarctic distribution, with at least one species breeding in any open, wet habitats in th ...
" 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 Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power of the ...
''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 A robber fly eating a hoverfly An insectivore is a carnivorous animal or plant that eats insects. An alternative term is entomophage, which can also refer to the human practice of eating insects. The first vertebrate insectivores were ...
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 Empirical evidence for a proposition is evidence, i.e. what supports or counters this proposition, that is constituted by or accessible to sense experience or experimental procedure. Empirical evidence is of central importance to the sciences and ...
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 mammalian whiskers (
vibrissae Vibrissae (; singular: vibrissa; ), more generally called Whiskers, are a type of stiff, functional hair used by mammals to sense their environment. These hairs are finely specialised for this purpose, whereas other types of hair are coarser ...
). Studies have shown that
Herbst corpuscle The corpuscles of Herbst or Herbst corpuscles are nerve-endings similar to the Pacinian corpuscle, found in the mucous membrane of the tongue, in pits on the beak and in other parts of the bodies of birds. They differ from Pacinian corpuscles in ...
s, 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 Calcification is the accumulation of calcium salts in a body tissue. It normally occurs in the formation of bone, but calcium can be deposited abnormally in soft tissue,Miller, J. D. Cardiovascular calcification: Orbicular origins. ''Nature Mat ...
projection on their beak, which they use to chip their way out of their
egg An egg is an organic vessel grown by an animal to carry a possibly fertilized egg cell (a zygote) and to incubate from it an embryo within the egg until the embryo has become an animal fetus that can survive on its own, at which point the a ...
. Commonly known as an egg tooth, this white spike is generally 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 A tooth ( : teeth) is a hard, calcified structure found in the jaws (or mouths) of many vertebrates and used to break down food. Some animals, particularly carnivores and omnivores, also use teeth to help with capturing or wounding prey, tear ...
, as the similarly-named projections of some
reptile Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocephalians ( ...
s are; instead, it is part of the
integumentary system The integumentary system is the set of organs forming the outermost layer of an animal's body. It comprises the skin and its appendages, which act as a physical barrier between the external environment and the internal environment that it serves ...
, as are
claw A claw is a curved, pointed appendage found at the end of a toe or finger in most amniotes (mammals, reptiles, birds). Some invertebrates such as beetles and spiders have somewhat similar fine, hooked structures at the end of the leg or tarsus ...
s and
scales Scale or scales may refer to: Mathematics * Scale (descriptive set theory), an object defined on a set of points * Scale (ratio), the ratio of a linear dimension of a model to the corresponding dimension of the original * Scale factor, a number w ...
. 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 The megapodes, also known as incubator birds or mound-builders, are stocky, medium-large, chicken-like birds with small heads and large feet in the family Megapodiidae. Their name literally means "large foot" and is a reference to the heavy legs ...
chicks have an egg tooth while still in the egg but lose it before hatching, while
kiwi Kiwi most commonly refers to: * Kiwi (bird), a flightless bird native to New Zealand * Kiwi (nickname), a nickname for New Zealanders * Kiwifruit, an edible berry * Kiwi dollar or New Zealand dollar, a unit of currency Kiwi or KIWI may also refe ...
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 Petrels are tube-nosed seabirds in the bird order Procellariiformes. Description The common name does not indicate relationship beyond that point, as "petrels" occur in three of the four families within that group (all except the albatross f ...
s keep theirs for nearly three weeks and marbled murrelets have theirs for up to a month. Generally, the egg tooth drops off, though in
songbird A songbird is a bird belonging to the suborder Passeri of the perching birds (Passeriformes). Another name that is sometimes seen as the scientific or vernacular name is Oscines, from Latin ''oscen'', "songbird". The Passeriformes contains 500 ...
s it is reabsorbed.


Color

The color of a bird's beak results from concentrations of
pigment A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Generally dyes are often organic compounds whereas pigments are often inorganic compo ...
s — primarily
melanin Melanin (; from el, μέλας, melas, black, dark) is a broad term for a group of natural pigments found in most organisms. Eumelanin is produced through a multistage chemical process known as melanogenesis, where the oxidation of the amino ...
s and
carotenoids Carotenoids (), also called tetraterpenoids, are yellow, orange, and red organic pigments that are produced by plants and algae, as well as several bacteria, and fungi. Carotenoids give the characteristic color to pumpkins, carrots, parsnips, ...
— in the epidermal layers, including the rhamphotheca.
Eumelanin Melanin (; from el, μέλας, melas, black, dark) is a broad term for a group of natural pigments found in most organisms. Eumelanin is produced through a multistage chemical process known as melanogenesis, where the oxidation of the amin ...
, 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 Melanin (; from el, μέλας, melas, black, dark) is a broad term for a group of natural pigments found in most organisms. Eumelanin is produced through a multistage chemical process known as melanogenesis, where the oxidation of the amino ...
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 whfich 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 In color theory, hue is one of the main properties (called Color appearance model#Color appearance parameters, color appearance parameters) of a color, defined technically in the CIECAM02 model as "the degree to which a Stimulus (physiology ...
of the color is determined by the precise mix of red and yellow pigments, while the
saturation Saturation, saturated, unsaturation or unsaturated may refer to: Chemistry * Saturation, a property of organic compounds referring to carbon-carbon bonds **Saturated and unsaturated compounds ** Degree of unsaturation **Saturated fat or fatty aci ...
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 and
diet Diet may refer to: Food * Diet (nutrition), the sum of the food consumed by an organism or group * Dieting, the deliberate selection of food to control body weight or nutrient intake ** Diet food, foods that aid in creating a diet for weight loss ...
. Colors are typically brightest as the breeding season approaches, and palest after breeding. Birds are capable of seeing colors in the
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...
range, and some species are known to have ultraviolet peaks of reflectance (indicating the presence of ultraviolet color) on their beaks. The presence and intensity of these peaks may indicate a bird's fitness, sexual maturity or pair bond status.
King King is the title given to a male monarch in a variety of contexts. The female equivalent is queen, which title is also given to the consort of a king. *In the context of prehistory, antiquity and contemporary indigenous peoples, the tit ...
and
emperor penguin The emperor penguin (''Aptenodytes forsteri'') is the tallest and heaviest of all living penguin species and is endemic to Antarctica. The male and female are similar in plumage and size, reaching in length and weighing from . Feathers of th ...
s, 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
conspecific Biological specificity is the tendency of a characteristic such as a behavior or a biochemical variation to occur in a particular species. Biochemist Linus Pauling stated that "Biological specificity is the set of characteristics of living organ ...
s. For instance, the very similarly-plumaged king and emperor penguins have UV-reflective spots in different positions on their beaks.


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 is an interaction in population ecology, whereby members of the same species compete for limited resources. This leads to a reduction in fitness for both individuals, but the more fit individual survives and is able to r ...
. For example, females of nearly all shorebirds have longer bills than males of the same species, and female
American avocet The American avocet (''Recurvirostra americana'') is a large wader in the avocet and stilt family, Recurvirostridae, that is found in North America. It spends much of its time foraging in shallow water or on mud flats, often sweeping its bill fro ...
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 Hornbills (Bucerotidae) are a family (biology), 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 coloured and sometimes has a Casque (an ...
s show
sexual dimorphism Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most ani ...
in the size and shape of both beaks and casques, and the female
huia The huia ( ; ; ''Heteralocha acutirostris'') is an extinct species of New Zealand wattlebird, endemic to the North Island of New Zealand. The last confirmed sighting of a huia was in 1907, although there was a credible sighting in 1924. It ...
'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 An androgen (from Greek ''andr-'', the stem of the word meaning "man") is any natural or synthetic steroid hormone that regulates the development and maintenance of male characteristics in vertebrates by binding to androgen receptors. This inc ...
s. For example, in
house sparrow The house sparrow (''Passer domesticus'') is a bird of the sparrow family Passeridae, found in most parts of the world. It is a small bird that has a typical length of and a mass of . Females and young birds are coloured pale brown and grey, a ...
s, melanins are produced only in the presence of
testosterone Testosterone is the primary sex hormone and anabolic steroid in males. In humans, testosterone plays a key role in the development of Male reproductive system, male reproductive tissues such as testes and prostate, as well as promoting secondar ...
;
castrated Castration is any action, surgical, chemical, or otherwise, by which an individual loses use of the testicles: the male gonad. Surgical castration is bilateral orchiectomy (excision of both testicles), while chemical castration uses pharmaceut ...
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 The black-headed gull (''Chroicocephalus ridibundus'') is a small gull that breeds in much of the Palearctic including Europe and also in coastal eastern Canada. Most of the population is migratory and winters further south, but some birds res ...
s and
indigo bunting The indigo bunting (''Passerina cyanea'') is a small seed-eating bird in the cardinal family, Cardinalidae. It is migratory, ranging from southern Canada to northern Florida during the breeding season, and from southern Florida to northern Sout ...
s.


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.


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 The garganey (''Spatula querquedula'') is a small dabbling duck. It breeds in much of Europe and across the Palearctic, but is strictly migratory, with the entire population moving to southern Africa, India (in particular Santragachi), Banglades ...
touches his beak to the blue speculum feathers on his wings in a fake preening display, and the male
Mandarin duck The mandarin duck (''Aix galericulata'') is a perching duck species native to the East Palearctic. It is medium-sized, at long with a wingspan. It is closely related to the North American wood duck, the only other member of the genus ''Aix''. ...
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 beak. The
platypus The platypus (''Ornithorhynchus anatinus''), sometimes referred to as the duck-billed platypus, is a semiaquatic, egg-laying mammal Endemic (ecology), endemic to Eastern states of Australia, eastern Australia, including Tasmania. The platypu ...
uses its bill to navigate underwater, detect food, and dig. The bill contains electroreceptors and mechanoreceptors, causing muscular contractions to help detect prey. It is one of the few species of mammals to use
electroreception Electroreception and electrogenesis are the closely-related biological abilities to perceive electrical stimuli and to generate electric fields. Both are used to locate prey; stronger electric discharges are used in a few groups of fishes to stu ...
.


Preening

The beak of birds plays a role in removing skin parasites (
ectoparasites Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has ...
) 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. Birds that have naturally deformed beaks have also been noted to have higher levels of parasites. 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. This overhang of the beak is thought to be under stabilising natural selection. Very long beaks are thought to be selected against because they are prone to a higher number of breaks, as has been demonstrated in rock pigeons. 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. Additionally, because of the role beaks play in preening, this is evidence for
coevolution In biology, coevolution occurs when two or more species reciprocally affect each other's evolution through the process of natural selection. The term sometimes is used for two traits in the same species affecting each other's evolution, as well ...
of the beak overhang morphology and body morphology of parasites. Artificially removing the ability to preen in birds, followed by readdition of preening ability was shown to result in changes in body size in lice. Once the ability of the birds to preen was reintroduced, the lice were found to show declines in body size suggesting they may evolve in response to preening pressures from birds who could respond in turn with changes in beak morphology.


Communication

A number of species, including
stork Storks are large, long-legged, long-necked wading birds with long, stout bills. They belong to the family called Ciconiidae, and make up the order Ciconiiformes . Ciconiiformes previously included a number of other families, such as herons an ...
s, some
owl Owls are birds from the order Strigiformes (), which includes over 200 species of mostly solitary and nocturnal birds of prey typified by an upright stance, a large, broad head, binocular vision, binaural hearing, sharp talons, and feathers a ...
s,
frogmouth The frogmouths are a group of nocturnal birds related to owlet-nightjars, swifts, and hummingbirds. Species in the group are distributed in the Indomalayan and Australasian realms. Biology They are named for their large flattened hooked bill and ...
s and the
noisy miner The noisy miner (''Manorina melanocephala'') is a bird in the honeyeater family, Meliphagidae, and is endemic to eastern and southeastern Australia. This miner is a grey bird, with a black head, orange-yellow beak and feet, a distinctive yellow ...
, 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 The toco toucan (''Ramphastos toco''), also known as the common toucan or giant 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 Sou ...
, which has the largest beak relative to the size of its body of any bird species, is capable of modifying the
blood flow Hemodynamics or haemodynamics are the dynamics of blood flow. The circulatory system is controlled by homeostatic mechanisms of autoregulation, just as hydraulic circuits are controlled by control systems. The hemodynamic response continuously m ...
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 New World sparrows are a group of mainly New World passerine birds, forming the family Passerellidae. They are seed-eating birds with conical bills, brown or gray in color, and many species have distinctive head patterns. Although they share t ...
s found in
salt marsh A salt marsh or saltmarsh, also known as a coastal salt marsh or a tidal marsh, is a coastal ecosystem in the upper coastal intertidal zone between land and open saltwater or brackish water that is regularly flooded by the tides. It is dominated ...
es along the North American coastlines show a strong correlation with summer temperatures recorded in the locations where the sparrows breed;
latitude In geography, latitude is a coordinate that specifies the north– south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north pol ...
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 A ratite () is any of a diverse group of flightless, large, long-necked, and long-legged birds of the infraclass Palaeognathae. Kiwi, the exception, are much smaller and shorter-legged and are the only nocturnal extant ratites. The systematics ...
s, including the
common ostrich The common ostrich (''Struthio camelus''), or simply ostrich, is a species of flightless bird native to certain large areas of Africa and is the largest living bird species. It is one of two extant species of ostriches, the only living members o ...
, the
emu The emu () (''Dromaius novaehollandiae'') is the second-tallest living bird after its ratite relative the ostrich. It is endemic to Australia where it is the largest native bird and the only extant member of the genus ''Dromaius''. The emu' ...
and the
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. It is one of the three living species of cassowary, alongside the dwarf c ...
, use various bare parts of their bodies (including their beaks) to dissipate as much as 40% of their metabolic heat production. 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.


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 In biology, a pair bond is the strong affinity that develops in some species between a mating pair, often leading to the production and rearing of offspring and potentially a lifelong bond. Pair-bonding is a term coined in the 1940s that is freque ...
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 Gannets are seabirds comprising the genus ''Morus'' in the family Sulidae, closely related to boobies. Gannets are large white birds with yellowish heads; black-tipped wings; and long bills. Northern gannets are the largest seabirds in the Nor ...
s raise their bills high and repeatedly clatter them, the male
puffin Puffins are any of three species of small alcids (auks) in the bird genus ''Fratercula''. 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 crev ...
nibbles at the female's beak, the male
waxwing The waxwings are three species of passerine birds classified in the genus ''Bombycilla''. They are pinkish-brown and pale grey with distinctive smooth plumage in which many body feathers are not individually visible, a black and white eyestripe, ...
puts his bill in the female's mouth and
raven A raven is any of several larger-bodied bird species of the genus ''Corvus''. These species do not form a single taxonomic group within the genus. There is no consistent distinction between "crows" and "ravens", common names which are assigned t ...
s hold each other's beaks in a prolonged "kiss". Armstrong (1965) p. 7 Billing can also be used as a gesture of appeasement or subordination. Subordinate
Canada jay The Canada jay (''Perisoreus canadensis''), also known as the gray jay, grey jay, camp robber, or whisky jack, is a passerine bird of the family Corvidae. It is found in boreal forests of North America north to the tree line, and in the Rock ...
routinely bill more dominant birds, lowering their body and quivering their wings in the manner of a young
bird food Bird food or bird seed is food (often varieties of seeds, nuts, and/or dried fruits) intended for consumption by wild and domestic birds. While most bird food is fed to commercial fowl (such as chicken or turkey), bird food is also used to fee ...
begging as they do so. A number of parasites, including rhinonyssids and ''
Trichomonas gallinae ''Trichomonas gallinae'' is a cosmopolitan parasite of birds including finches, pigeons, doves, turkeys, chickens, parrots, raptors (hawks, golden eagle, etc.). The condition in birds of prey is called frounce.Kennedy, Murry J. "Trichomoniasis ...
'' are known to be transferred between birds during episodes of billing. Use of the term extends beyond avian behavior; "billing and cooing" in reference to human courtship (particularly kissing) has been in use since
Shakespeare's William Shakespeare ( 26 April 1564 – 23 April 1616) was an English playwright, poet and actor. He is widely regarded as the greatest writer in the English language and the world's pre-eminent dramatist. He is often called England's nation ...
time, and derives from the courtship of doves.


Beak trimming

Because the beak is a sensitive organ with many sensory receptors, beak trimming (sometimes referred to as 'debeaking') is "acutely painful" to the birds it is performed on. It is nonetheless routinely done to intensively farmed
poultry Poultry () are domesticated birds kept by humans for their eggs, their meat or their feathers. These birds are most typically members of the superorder Galloanserae (fowl), especially the order Galliformes (which includes chickens, quails, a ...
flocks, particularly
laying Laying is the act of making equipment level. It usually involves moving equipment in small motions so that spirit levels are centralised in all planes. Movement is usually done by small worm gears or other fine setting devices for accurate sma ...
and
broiler A broiler is any chicken (''Gallus gallus domesticus'') that is bred and raised specifically for meat production. Most commercial broilers reach slaughter weight between four and six weeks of age, although slower growing breeds reach slaughte ...
breeder flocks, because it helps reduce the damage the flocks inflict on themselves due to a number of
stress Stress may refer to: Science and medicine * Stress (biology), an organism's response to a stressor such as an environmental condition * Stress (linguistics), relative emphasis or prominence given to a syllable in a word, or to a word in a phrase ...
-induced behaviors, including
cannibalism Cannibalism is the act of consuming another individual of the same species as food. Cannibalism is a common ecological interaction in the animal kingdom and has been recorded in more than 1,500 species. Human cannibalism is well documented, b ...
,
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.Sherwin, C.M., (2010). The welfare and ethical asses ...
and
feather pecking Feather pecking is a behavioural problem that occurs most frequently amongst domestic hens reared for egg production,Huber-Eicher, B. and Sebo, F. 2001. The prevalence of feather pecking and development in commercial flocks of laying hens. Applie ...
. 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 A neuroma (; plural: neuromata or neuromas) is a growth or tumor of nerve tissue. Neuromas tend to be benign tumor, benign (i.e. not malignant, cancerous); many Nervous tissue#Tumours, nerve tumors, including those that are commonly malignant, ar ...
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 The adrenal glands (also known as suprarenal glands) are endocrine glands that produce a variety of hormones including adrenaline and the steroids aldosterone and cortisol. They are found above the kidneys. Each gland has an outer cortex which ...
s weigh less, and their plasma
corticosterone Corticosterone, also known as 17-deoxycortisol and 11β,21-dihydroxyprogesterone, is a 21-carbon steroid hormone of the corticosteroid type produced in the cortex of the adrenal glands. It is of minor importance in humans, except in the very rare ...
levels are lower than those found in untrimmed poultry, indicating that they are less stressed overall. A similar but separate practice, usually performed by an
avian veterinarian An avian veterinarian is a veterinarian who specializes in treating birds. All veterinarians, upon first qualification from a certified veterinary college, may treat any species. Additional training is required for qualification to become a recogni ...
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. Amongst
raptor Raptor or RAPTOR may refer to: Animals The word "raptor" refers to several groups of bird-like dinosaurs which primarily capture and subdue/kill prey with their talons. * Raptor (bird) or bird of prey, a bird that primarily hunts and feeds on v ...
keepers, this practice is commonly known as "coping".


Bill tip organ

The bill tip organ is a region found near the tip of the bill in several types of birds that forage particularly by probing. The region has a high density of nerve endings known as the corpuscles of Herbst. This consists of pits in the bill surface which in the living bird is occupied by cells that sense pressure changes. The assumption is that this allows the bird to perform 'remote touch', which means that it can detect movements of animals which the bird does not directly touch. Bird species known to have a 'bill-tip organ' include
ibis The ibises () (collective plural ibis; classical plurals ibides and ibes) are a group of long-legged wading birds in the family Threskiornithidae, that inhabit wetlands, forests and plains. "Ibis" derives from the Latin and Ancient Greek word f ...
is, shorebirds of the family
Scolopacidae Sandpipers are a large family, Scolopacidae, of waders. They include many species called sandpipers, as well as those called by names such as curlew and snipe. The majority of these species eat small invertebrates picked out of the mud or soil. ...
, and
kiwi Kiwi most commonly refers to: * Kiwi (bird), a flightless bird native to New Zealand * Kiwi (nickname), a nickname for New Zealanders * Kiwifruit, an edible berry * Kiwi dollar or New Zealand dollar, a unit of currency Kiwi or KIWI may also refe ...
s. There is a suggestion that across these species, the bill tip organ is better-developed among species foraging in wet habitats (water column, or soft mud) than in species using a more terrestrial foraging. However, it has been described in terrestrial birds too, including
parrot Parrots, also known as psittacines (), are birds of the roughly 398 species in 92 genera comprising the order Psittaciformes (), found mostly in tropical and subtropical regions. The order is subdivided into three superfamilies: the Psittacoid ...
s, who are known for their dextrous extractive foraging techniques. Unlike probing foragers, the tactile pits in parrots are embedded in the hard
keratin Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. Alpha-keratin (α-keratin) is a type of keratin found in vertebrates. It is the key structural material making up scales, hair, nails, feathers, ho ...
(or
rhamphotheca 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 eating, preening, manipulating objects, killing prey, fighting, probing for food, ...
) of the bill, rather than the bone, and along the inner edges of the curved bill, rather than being on the outside of the bill.


See also

* * *


Footnotes


References


Bibliography

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