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The beak, bill, and/or rostrum is an external anatomical structure found mostly in
birds Birds are a group of warm-blooded vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With ...

birds
, but also in non-avian
dinosaurs Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic Geological period, period, between 243 and 233.23 annum, million years ago, although the exact origin and timing of the evolution o ...
and some mammals. A beak is used for eating and for preening, manipulating objects, killing prey, fighting, probing for food,
courtship Courtship is the period of development towards a sexual relationship An intimate relationship is an interpersonal relationship that involves Physical intimacy, physical or emotional intimacy. Although an intimate relationship is commonly a ...
and feeding young. The terms ''beak'' and '' rostrum'' are also used to refer to a similar mouth part in some
ornithischia Ornithischia () is an extinct Order (biology), order of mainly Herbivore, herbivorous dinosaurs characterized by a pelvic structure superficially similar to that of birds. The name ''Ornithischia'', or "bird-hipped", reflects this similarity and is ...

ornithischia
ns,
pterosaur Pterosaurs (; from Greek ''pteron'' and ''sauros'', meaning "wing lizard") were flying reptiles of the extinct clade or Order (biology), order Pterosauria. They existed during most of the Mesozoic: from the late Triassic to the end of the Cretac ...
s,
turtle Turtles are an of s known as Testudines, characterized by a developed mainly from their ribs. Modern turtles are divided into two major groups, the s and which differ in the way the head retracts. There are 360 living and recently extinct ...

turtle
s,
cetacea Cetaceans (from la, cetus Cetus () is a constellation, sometimes called 'the whale' in English. The Cetus (mythology), Cetus was a sea monster in Greek mythology which both Perseus and Heracles needed to slay. Cetus is in the region of the ...

cetacea
ns,
dicynodont Dicynodontia is a taxon of anomodont therapsids with beginnings in the Guadalupian, mid-Permian, which were dominant in the Permian, Late Permian, survived the Permian–Triassic extinction event, Permian Extinction that wiped out most other therap ...

dicynodont
s,
anuran A frog is any member of a diverse and largely Carnivore, carnivorous group of short-bodied, tailless amphibians composing the order (biology), order Anura (literally ''without tail'' in Ancient Greek). The oldest fossil "proto-frog" ''Triadoba ...
tadpole A tadpole is the larval stage A larva (plural larvae ) is a distinct juvenile form many animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With ...
s,
monotreme Monotremes are prototherian mammals of the order Monotremata. They are one of the three main 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 Monotremes are one of the three main groups of living mammal Mammals (from Latin language, Latin , 'breast') are a group of vertebrate animals constitutin ...

echidna
s and
platypus The platypus (''Ornithorhynchus anatinus''), sometimes referred to as the duck-billed platypus, is a semiaquatic In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical str ...

platypus
es, which have a beak-like structure),
siren
siren
s,
pufferfish Tetraodontidae is a family In human society A society is a Social group, group of individuals involved in persistent Social relation, social interaction, or a large social group sharing the same spatial or social territory, typ ...

pufferfish
,
billfish The term billfish refers to a group of predatory fish Predatory fish are fish Fish are Aquatic animal, aquatic, craniate, gill-bearing animals that lack Limb (anatomy), limbs with Digit (anatomy), digits. They form a sister group to the t ...
es and
cephalopods A cephalopod is any member of the mollusca Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda. The members are known as molluscs or mollusks (). Around 85,000 extant taxon, extant species of molluscs are ...
. 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 known as 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, also known as raptors, include species of bird Birds are a group of s constituting the Aves , characterised by s, toothless beaked jaws, the of eggs, a high rate, a four-chambered , and a strong yet lightweight . Bi ...
, in modern
ornithology Ornithology is a branch of zoology Zoology ()The pronunciation of zoology as is usually regarded as nonstandard, though it is not uncommon. is the branch of biology that studies the Animal, animal kingdom, including the anatomy, structure, ...

ornithology
, 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) was a form of the English language English is a West Germanic language of the Indo-European language family The Indo-European languages are a language family A language is a structured sys ...
''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: 3-space or, rarely, tri-dimensional space) is a geometric setting in which three values (called parameter A parameter (from the Ancient Greek language, Ancient Greek wikt:παρά#Ancient Greek, παρά, ''par ...
network of bony spicules (or
trabecula A trabecula (plural trabeculae, from Latin for "small beam") is a small, often microscopic, tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', ...
e) seated in soft
connective tissue Connective tissue is one of the many basic types of animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions ...
and surrounded by the hard outer layers of the beak. The avian jaw apparatus is made up of two units: one
four-bar linkageA four-bar linkage, also called a four-bar, is the simplest movable closed-chain linkage. It consists of four bodies, called bars or links, connected in a loop by four joints. Generally, the joints are configured so the links move in parallel planes ...

four-bar linkage
mechanism and one five-bar linkage mechanism.


Mandibles

The upper mandible is supported by a three-pronged
bone A bone is a rigid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North America * ''Triphosa dubit ...

bone
called the intermaxillary. The upper prong of this bone is embedded into the forehead, while the two lower prongs attach to the sides of the
skull The skull is a bone A bone is a rigid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North A ...

skull
. At the base of the upper mandible a thin sheet of nasal bones is attached to the skull at the nasofrontal hinge, which gives mobility to the upper mandible, allowing it to move upwards and downwards.Proctor and Lynch (1998), p. 66. 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 mostly flightless s are a well-known example of flightless birds. Flightless birds are birds that through evolution lost the ability to flight, fly. There are over 60 extant species, including the well ...

ratite
s. Here, the
vomer The vomer () is one of the unpaired facial skeleton, facial bones of the human skull, skull. It is located in the midsagittal line, and Articulation (anatomy), articulates with the sphenoid bone, sphenoid, the ethmoid bone, ethmoid, the left and r ...

vomer
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
proximal Standard anatomical terms of location deal unambiguously with the anatomy of animals, including humans. Terms used generally derive from Latin or Greek language, Greek roots and used to describe something in its standard anatomical position. This ...
ly, 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.Gill (1995), p. 148. 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 In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecul ...
, which have well-developed
digastric muscle The digastric muscle (also digastricus) (named ''digastric'' as it has two 'bellies') is a small muscle Muscle is a soft tissue found in most animals. Muscle cells contain protein Proteins are large biomolecules or macromolecules that a ...
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 horny sheath of
keratin Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. Alpha-keratin Alpha-keratin, or α-keratin, is a type of keratin Keratin () is one of a family of fibrous structural proteins known as Scleroprot ...

keratin
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 of the bird's
epidermis The epidermis is the outermost of the three layers that comprise the skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate Vertebrates () comprise all species of animal Animals (also calle ...
,Campbell and Lack (1995), p. 47. growing from plates at the base of each mandible. There is a
vascular The blood vessels are the components of the circulatory system The circulatory system, also called the cardiovascular system or the vascular system, is an Biological system, organ system that permits blood to circulate and transport nutrien ...
layer between the rhamphotheca and the deeper layers of the
dermis The dermis or corium is a layer of skin between the epidermis (skin), epidermis (with which it makes up the cutis (anatomy), cutis) and subcutaneous tissues, that primarily consists of dense irregular connective tissue and cushions the body from s ...
, which is attached directly to the
periosteum The periosteum is a membrane Image:Schematic size.jpg, up150px, Schematic of size-based membrane exclusion A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other sma ...
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, 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 Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the endling, last individual o ...
birds have a single seamless rhamphotheca, species in a few families, including the
albatross Albatrosses are very large Seabird, seabirds in the family (biology), family Diomedeidae. They range widely in the Southern Ocean and the North Pacific Ocean, Pacific. They are absent from the North Atlantic Ocean, Atlantic, although fossil rem ...

albatross
es and the
emu The emu (''Dromaius novaehollandiae'') is the second-largest living bird by height, after its ratite relative, the ostrich. It is endemic Endemism is the state of a species In biology, a species is the basic unit of biological c ...

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 A aggregate accessory fruit">aggregate_fruit.html" ;"title="strawberry aggregate fruit">aggregate accessory fruit damaged by a mouse eating the seeds (achenes). Seed predation, often referred to as granivory, is a type of plant-animal interacti ...
(seed-eating) birds, for example, have ridges in their tomia, which help the bird to slice through a seed's outer
hull Hull may refer to: Structures * Chassis, of an armored fighting vehicle * Fuselage, of an aircraft * Hull (botany), the outer covering of seeds * Hull (watercraft), the body or frame of a ship * Submarine hull Mathematics * Affine hull, in affin ...
. Most
falcon Falcons () are birds of prey Birds of prey, also known as raptors, include species of bird that primarily hunt and feed on vertebrates that are large relative to the hunter. Additionally, they have Bird vision, keen eyesight for detecting fo ...

falcon
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. ...

kites
, principally those that prey on insects or lizards, also have one or more of these sharp projections, as do the
shrike Shrikes () are carnivore, carnivorous passerine birds of the family Laniidae. The family is composed of 34 species in four genus, genera. The family name, and that of the largest genus, ''Lanius'', is derived from the Latin word for "butcher", an ...

shrike
s. 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 Ancient Rome, Roman authors to refer to an unspecified waterbird. The common merga ...
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 Asilidae, robber fly eating a hoverfly File:Myresluger2.jpg, The giant anteater, a large insectivorous mammal An insectivore is a Carnivore, carnivorous plant or animal that eats insects. An alternative term is entomophage, which also refer ...
(preferring hard-shelled prey) or
snail A snail is, in loose terms, a shelled gastropod The gastropods (), commonly known as snails and slugs, belong to a large taxonomic Taxonomy (general) is the practice and science of classification of things or concepts, including the pr ...

snail
eaters, and the brush-like projections may help to increase the
coefficient of friction Friction is the force In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion (physics), ...
between the mandibles, thereby improving the bird's ability to hold hard prey items. Serrations on
hummingbird Hummingbirds are Bird, birds native to the Americas and comprise the Family (biology), biological family Trochilidae. With about 360 species, they occur from Alaska to Tierra del Fuego, but the vast majority of the species are found in the tropi ...

hummingbird
bills, found in 23% of all hummingbird genera, may perform a similar function, allowing the birds to effectively hold insect prey. They may also allow shorter-billed hummingbirds to function as , 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 A goose (plural geese) is a bird of any of several waterfowl species in the family Anatidae. This group comprises the genera '' Anser'' (the grey geese and white geese) and ' ...
, for example, has a reddish-pink bill with black tomia, while the whole beak of the similar
Ross's goose Ross's goose (''Anser rossii'') is a white goose with black wingtips and a relatively short neck, and 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 40% sma ...
is pinkish-red, without darker tomia.


Culmen

The culmen is the
dorsal Dorsal (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Repu ...
ridge of the upper mandible. Likened by ornithologist
Elliott Coues Elliott Ladd Coues (; September 9, 1842 – December 25, 1899) was an American army surgeon, historian, ornithologist Ornithology is a branch of zoology Zoology ()The pronunciation of zoology as is typically regarded as nonstandard, t ...
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 Despite the various English dialects spoken from country to country and within different regions of the same country, there are only slight regional variations in English orthography English orthography is th ...
. 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. Etymology The scientific name is from Ancient Greek. ''Loxia'' derives from ''loxos'', "crosswise", and ''pytyopsittacus'' is from ''pit ...
is strongly decurved, while that of the very similar
red crossbill The red crossbill or common crossbill (''Loxia curvirostra'') is a small passerine A passerine is any bird of the Order (biology), order Passeriformes (, Latin ''passer'' (“sparrow”) + ''formis'' (“-shaped”)), which includes more than ...
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 (biology), family of birds. Reproduction, Breeding adults have a plumage that includes a broad black head and neck with a greenish, purplis ...

common loon
is all dark, while that of the very similarly
plumage Plumage ( "feather") is a layer of feather Feathers are epidermal growths that form a distinctive outer covering, or plumage Plumage ( "feather") is a layer of feather Feathers are epidermal growths that form a distinctive outer ...
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 Loons (North America) or divers (United Kingdom / Ireland) are a group of aquatic bird Birds are a group of warm-blood ...
is pale towards the tip.


Gonys

The gonys is the
ventral Standard anatomical terms of location deal unambiguously with the of s, including s. Terms used generally derive from or roots and used to describe something in its . This position provides a definition of what is at the front ("anterior"), be ...

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.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 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.


Gape

In
bird anatomy Bird anatomy, or the physiology, physiological structure of birds' bodies, shows many unique adaptations, mostly aiding bird flight, flight. Birds have a light skeletal system and light but powerful musculature which, along with circulatory and ...
, 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 Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any ind ...

mandible
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 Altricial young birds In biology, altricial species are those in which the young are underdeveloped at the time of birth, but with the aid of their parents develop in spurts after birth. The word is derived from the Latin Latin (, or , ) is a ...
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 The cuckoo-finch (''Anomalospiza imberbis''), also known as the parasitic weaver or cuckoo weaver, is a small passerine bird now placed in the family Viduidae with the indigob ...
and
Estrildidae Estrildidae, or estrildid finches, is a family In human society, family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of familie ...
, 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 Birds are a group of warm-blooded vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), bi ...
species found that the gapes were conspicuous in the
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, ...

ultraviolet
spectrum (visible to birds but not to humans). Parents may, however, not rely solely on the gape coloration, and other factors influencing their decision remain unknown. Red gape color has been shown in several experiments to induce feeding. An experiment in manipulating brood size and immune system with
barn swallow The barn swallow (''Hirundo rustica'') is the most widespread species of swallow The swallows, martins, and saw-wings, or Hirundinidae, are a family of passerine A passerine is any bird of the Order (biology), order Passeriformes (, Lat ...

barn swallow
nestlings showed the vividness of the gape was positively
correlated In , correlation or dependence is any statistical relationship, whether or not, between two s or . In the broadest sense correlation is any statistical association, though it actually refers to the degree to which a pair of variables are rel ...

correlated
with
T-cell A T cell is a type of lymphocyte A lymphocyte is a type of white blood cell in the immune system The immune system is a network of biological processes that protects an organism In biology, an organism (from Ancient Greek, Greek: ...
–mediated
immunocompetence Immunocompetence is the ability of the body to produce a normal immune response An immune response is a reaction which occurs within an organism for the purpose of defending against foreign invaders. These invaders include a wide variety of differen ...
, and that larger brood size and injection with an
antigen In immunology Immunology is a branch of biology that covers the study of immune systems in all organisms. Immunology charts, measures, and contextualizes the Physiology, physiological functioning of the immune system in states of both health ...
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 The cuckoos are a family of bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves , characterised by feathers, toothless beak ...

common cuckoo
(''Cuculus canorus'') did not induce extra feeding in host parents. Some
brood parasite Brood parasites are organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interact ...
s, such as the Hodgson's hawk-cuckoo (''C. fugax''), have colored patches on the wing that mimic the gape color of the parasitized species. When born, the chick's gape flanges are fleshy. As it grows into a fledgling, the gape flanges remain somewhat swollen and can thus be used to recognize that a particular bird is young. By the time it reaches adulthood, the gape flanges will no longer be visible.


Nares

Most species of birds have external nares (
nostril A nostril (or naris , plural ''nares'' ) is either of the two orifices of the nose A nose is a protuberance in vertebrates that houses the nostrils, or nares, which receive and expel air for Respiration (physiology), respiration alongside ...
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 A nose is a protuberance in vertebrates that houses the nostrils, or nares, which receive and expel air for Respiration (physiology), respiration alongside the mouth. Beh ...
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 A biological system is a complex network Network and networking may refer to: Arts, entertainment, and media * ''Network'' (1976 film), a 1976 Ame ...

respiratory system
.Campbell and Lack (1985), p. 375. In most bird species, the nares are located in the basal third of the upper mandible.
Kiwi KIWI (102.9 FM, "Radio Lobo") is a commercial radio station , Sweden , Norway Radio broadcasting is transmission of audio signal, audio (sound), sometimes with related metadata, by radio waves intended to reach a wide audience. In terrestria ...
s 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 In human society, family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of families is to maintain ...

Cormorant
s and
darter The darters, anhingas, or snakebirds are mainly tropical The tropics are the region of Earth surrounding the Equator. They are delimited in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in ...

darter
s have primitive external nares as nestlings, but these close soon after the birds
fledge Fledging is the stage in a flying animal A number of animals are capable of aerial locomotion, either by powered flight or by gliding (flight), gliding. This trait has appeared by evolution many times, without any single common ancestor. Fl ...
; adults of these species (and
gannet Gannets are seabirds Seabirds (also known as marine birds) are bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves , characterised by feathers, toothless beaked jaws, the Oviparity, layi ...

gannet
s and of all ages, which also lack external nostrils) breathe through their mouths. There is typically a
septum In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanis ...
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 Falcons () are birds of prey Birds of prey, also known as raptors, include species of bird that primarily hunt and feed on vertebrates that are large relative to the hunter. Additionally, they have Bird vision, keen eyesight for detecting fo ...

falcon
s, have a small bony
tubercule Image:Mammillaria marksiana.jpg, 250px, This view of the cactus ''Mammillaria marksiana'' shows its pattern of prominent tubercles, with the spines emanating from each tubercle's tip. In anatomy, a tubercle (literally 'small tuber', Latin for 'lum ...
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 (cartilaginous tissue) is a resilient and smooth elastic tissue Elastic is a word often used to describe or identify certain types of elastomer An elastomer is a polymer A polymer (; Greek ''wikt:poly-, poly-'', "many" + ''wikt:-m ...

cartilaginous
flap.Gill (1995), p. 117. 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 frogmouths are covered with large dome-shaped opercula, which help to reduce the rapid evaporation of water vapor, and may also help to increase condensation within the nostrils themselves—both critical functions, since the nestlings get fluids only from the food their parents bring them. 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'', this is a different structure. Tapaculos are the only birds known to have the ability to move their opercula.


Rosette

Some species, such as the puffin, 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 (language), Latin ''cera'', which means "wax") or ceroma which covers the base of their bill. This structure typically contains the Beak#Nares, nares, except in the owls, where the nares are Anatomical terms of location#Proximal and distal, 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 Montagu'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 (UV) is a form of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, ...

ultraviolet
(UV) component, with a UV peak that correlates to the bird's mass. A chick with a lower body mass has a UV peak at a higher wavelength than a chick with a higher body mass does. Studies have shown that parent owls preferentially feed chicks with ceres that show higher wavelength UV peaks, that is, lighter-weight chicks. The color or appearance of the cere can be used to distinguish between males and females in some species. For example, the male great curassow has a yellow cere, which the female (and young males) lack. The male budgerigar's cere is royal blue, while the female's is a very pale blue, white, or brown.


Nail

All birds of the family Anatidae (ducks, geese, and swans) have a nail, a plate of hard horny tissue at the tip of the beak. This shield-shaped structure, which sometimes spans the entire width of the beak, is often bent at the tip to form a hook. It serves different purposes depending on the bird's primary food source. Most species use their nails to dig seeds out of mud or vegetation, while diving ducks use theirs to pry molluscs from rocks. There is evidence that the nail may help a bird to grasp things; species which use strong grasping motions to secure their food (such as when catching and holding onto a large squirming frog) have very wide nails. Certain types of mechanoreceptors, nerve cells that are sensitive to pressure, vibration or touch, are located under the nail. The shape or color of the nail can sometimes be used to help distinguish between similar-looking species or between various ages of waterfowl. For example, the greater scaup has a wider black nail than does the very similar lesser scaup. Juvenile "Grey goose, 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 (language), 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 mammalian whiskers (vibrissae). 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 (biology), egg.Campbell and Lack (1985), p. 178. 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, as the similarly-named projections of some reptiles are; instead, it is part of the integumentary system, as are claws and scale (zoology), scales. The hatching chick first uses its egg tooth to break the membrane around an air chamber at the wide end of the egg. Then it pecks at the eggshell while turning slowly within the egg, eventually (over a period of hours or days) creating a series of small circular fractures in the shell.Gill (1995), p. 427. 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.Gill (1995), p. 428. 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 (bird), kiwi 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 petrels keep theirs for nearly three weeks and marbled murrelets have theirs for up to a month. Generally, the egg tooth drops off, though in songbirds it is reabsorbed.


Color

The color of a bird's beak results from concentrations of pigments—primarily melanins 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.Hill (2010), p. 62. 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 of the color is determined by the precise mix of red and yellow pigments, while the saturation (color theory), saturation 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 hormone, hormonal state and diet (nutrition), diet. Colors are typically brightest as the breeding season approaches, and palest after breeding. Birds are capable of bird vision, seeing colors in the
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, ...

ultraviolet
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 penguin, King and emperor penguins, for example, show spots of ultraviolet reflectance only as adults. These spots are brighter on paired birds than on courting birds. The position of such spots on the beak may be important in allowing birds to identify conspecifics. For instance, the very similarly-plumaged king and emperor penguins have UV-reflective spots in different positions on their beaks.


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. For example, females of nearly all shorebirds have longer bills than males of the same species, and female American avocets have beaks which are slightly more upturned than those of males. Males of the larger gull species have bigger, stouter beaks than those of females of the same species, and immatures can have smaller, more slender beaks than those of adults. Many hornbills show sexual dimorphism in the size and shape of both beaks and Casque (anatomy), casques, and the female huia'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 androgens. For example, in house sparrows, melanins are produced only in the presence of testosterone; castration, castrated 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 gulls and indigo buntings.


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 touches his beak to the blue speculum feathers on his wings in a fake preening display, and the male Mandarin duck does the same with his orange sail feathers. 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 In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical str ...

platypus
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.


Preening

The beak of birds plays a role in removing skin parasites (Ectoparasitic infestation, ectoparasites) such as lice. It is mainly the tip of the beak that does this. Studies have shown that inserting a bit to stop birds from using the tip results in increased parasite loads in pigeons. 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 Stabilizing selection, 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 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 storks, some owls, frogmouths and the noisy miner, 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, which has the largest beak relative to the size of its body of any bird species, is capable of modifying the blood flow to its beak. This process allows the beak to work as a "transient thermal radiator", reportedly rivaling an elephant#Ears, elephant's ears in its ability to radiate body heat. Measurements of the bill sizes of several species of American sparrows found in salt marshes along the North American coastlines show a strong correlation with summer temperatures recorded in the locations where the sparrows breed; latitude 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 mostly flightless s are a well-known example of flightless birds. Flightless birds are birds that through evolution lost the ability to flight, fly. There are over 60 extant species, including the well ...

ratite
s, including the common ostrich, the
emu The emu (''Dromaius novaehollandiae'') is the second-largest living bird by height, after its ratite relative, the ostrich. It is endemic Endemism is the state of a species In biology, a species is the basic unit of biological c ...

emu
and the southern cassowary, 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 bonding. 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. Gannets raise their bills high and repeatedly clatter them, the male puffin nibbles at the female's beak, the male waxwing puts his bill in the female's mouth and ravens hold each other's beaks in a prolonged "kiss". Billing can also be used as a gesture of appeasement or subordination. Subordinate Canada jay routinely bill more dominant birds, lowering their body and quivering their wings in the manner of a young bird food begging as they do so. A number of parasites, including Rhinonyssidae, rhinonyssids and ''Trichomonas gallinae'' 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 William Shakespeare, Shakespeare's 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 flocks, particularly layer hen, laying and broiler breeder flocks, because it helps reduce the damage the flocks inflict on themselves due to a number of stress (biology), stress-induced behaviors, including cannibalism (zoology), cannibalism, vent pecking and feather pecking. 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 glands weigh less, and their plasma corticosterone 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 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 Bird of prey, raptor 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 ibisis, shorebirds of the family Scolopacidae, and Kiwi (bird), kiwis. There is a suggestion that across these species, the bill tip organ is more well 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 parrots, 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 Alpha-keratin, or α-keratin, is a type of keratin Keratin () is one of a family of fibrous structural proteins known as Scleroprot ...

keratin
(or Beak#Rhamphotheca, rhamphotheca) 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

* Bird anatomy * Rostrum (anatomy) * Snout


References


Sources

* * * * * * * * * * * * * {{Authority control Parts of a bird beak, Articles containing video clips