Beak
The beak, bill, or rostrum is an external anatomical structure found mostly in
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
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 maxilla (upper) and mandible (lower).[4](p147) The upper, and in some cases the lower, mandibles are strengthened internally by a complex three-dimensional network of bony spicules (or trabeculae) seated in soft connective tissue and surrounded by the hard outer layers of the beak.[5](p149)[6] The avian jaw apparatus is made up of two units: one four-bar linkage mechanism and one five-bar linkage mechanism.[7]
Mandibles
The upper mandible is supported by a three-pronged bone called the intermaxillary. The upper prong of this bone is embedded into the forehead, while the two lower prongs attach to the sides of the skull. At the base of the upper mandible a thin sheet of nasal bones is attached to the skull at the nasofrontal hinge, which gives mobility to the upper mandible, allowing it to move upward and downward.[2]
The base of the upper mandible, or the roof when seen from the mouth, is the palate, the structure of which differs greatly in the ratites. Here, the vomer 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.[a]
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
Rhamphotheca
The outer surface of the beak consists of a thin sheath of keratin called the rhamphotheca,[2][5](p148) which can be subdivided into the rhinotheca of the upper mandible and the gnathotheca of the lower mandible.[10](p47) This covering arises from the Malpighian layer of the bird's epidermis,[10](p47)
growing from plates at the base of each mandible.
While most extant birds have a single seamless rhamphotheca, species in a few families, including the albatrosses[10](p47) and the emu, have compound rhamphothecae that consist of several pieces separated and defined by softer keratinous grooves.[17] 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.[18]
Tomia
The tomia (singular tomium) are the cutting edges of the two mandibles.[10](p598) 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.[19]
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
Culmen
The culmen is the
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 is strongly decurved, while that of the very similar red crossbill is more moderately curved.[29] The culmen of a juvenile common loon is all dark, while that of the very similarly plumaged juvenile yellow-billed loon is pale towards the tip.[30]
Gonys
The gonys is the
Commissure
Depending on its use, commissure may refer to the junction of the upper and lower mandibles,[4](p155) or alternately, to the full-length apposition of the closed mandibles, from the corners of the mouth to the tip of the beak.[10](p105)
Gape
In bird anatomy, the gape is the interior of the open mouth of a bird, and the gape flange is the region where the two mandibles join together at the base of the beak.[33] The width of the gape can be a factor in the choice of food.[34]
Gapes of juvenile
Red gape color has been shown in several experiments to induce feeding. An experiment in manipulating brood size and immune system with
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.[42] By the time it reaches adulthood, the gape flanges will no longer be visible.
Nares
Most species of birds have external nares (
Species in the bird order
Operculum
The nares of some birds are covered by an operculum (plural opercula), a membraneous, horny or cartilaginous flap.[5](p117)[47] In diving birds, the operculum keeps water out of the nasal cavity;[5](p117) when the birds dive, the impact force of the water closes the operculum.[48] Some species which feed on flowers have opercula to help to keep pollen from clogging their nasal passages,[5](p117) while the opercula of the two species of Attagis seedsnipe help to keep dust out.[49] 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.[50] In
Rosette
Some species, such as the puffin, have a fleshy rosette, sometimes called a "gape rosette",[51] at the corners of the beak. In the puffin, this is grown as part of its display plumage.[52]
Cere
Birds from a handful of families—including raptors, owls, skuas, parrots, turkeys and curassows—have a waxy structure called a cere (from the
which covers the base of their bill. This structure typically contains the nares, except in the owls, where the nares are distal to the cere. Although it is sometimes feathered in parrots,[55] the cere is typically bare and often brightly colored.[19] 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.[56] The cere color of youngThe 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.[58] The male budgerigar's cere is royal blue, while the female's is a very pale blue, white, or brown.[59]
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.[60] This shield-shaped structure, which sometimes spans the entire width of the beak, is often bent at the tip to form a hook.[61] 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,[62] while
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.[66] Juvenile "
Rictal bristles
Rictal bristles are stiff hair-like feathers that arise around the base of the beak.[68] They are common among
Egg tooth
Full-term chicks of most bird species have a small sharp,
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.[71] 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,[5](p427) while kiwi chicks never develop one; chicks of both families escape their eggs by kicking their way out.[73] Most chicks lose their egg teeth within a few days of hatching,[10](p178) though petrels keep theirs for nearly three weeks[5](p428) and marbled murrelets have theirs for up to a month.[74] Generally, the egg tooth drops off, though in songbirds it is resorbed.[5](p428)
Color
The color of a bird's beak results from concentrations of
The
Birds are capable of seeing colors in the ultraviolet range, and some species are known to have ultraviolet peaks of reflectance (indicating the presence of ultraviolet color) on their beaks.[78] The presence and intensity of these peaks may indicate a bird's fitness,[56] sexual maturity or pair bond status. [78]
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.[79] For example, females of nearly all shorebirds have longer bills than males of the same species,[80] and female American avocets have beaks which are slightly more upturned than those of males.[81] 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.[82] Many hornbills show sexual dimorphism in the size and shape of both beaks and casques, and the female huia's slim, decurved bill was nearly twice as long as the male's straight, thicker one.[10](p48)
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; 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.[83]
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.[84]
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.[85][86] 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.[87] 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.[88] 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.[85][86]
Functions
Eating
Different species' beaks have evolved according to their diet; for example,
Self-defensive pecking
Birds may bite or stab with their beaks to defend themselves.[89]
Displays (for courtship, territoriality, or deterrence)
Some species use their beaks in displays of various sorts. As part of his courtship, for example, the male
Sensory detection
The
Preening
The beak of birds plays a role in removing skin parasites (ectoparasites) such as lice. It is mainly the tip of the beak that does this. Studies have shown that inserting a bit to stop birds from using the tip results in increased parasite loads in pigeons.[92] Birds that have naturally deformed beaks have also been noted to have higher levels of parasites.[93][94][95][96] 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.[92]
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.[97] 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.[98] The same pattern has been seen in surveys of Peruvian birds.[99]
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[92] who could respond in turn with changes in beak morphology.[92]
Communicative percussion
A number of species, including
Heat exchange
Studies have shown that some birds use their beaks to rid themselves of excess heat. The
Measurements of the bill sizes of several species of
Billing
During courtship, mated pairs of many bird species touch or clasp each other's bills. Termed billing (also nebbing in British English),[104] this behavior appears to strengthen pair bonding.[105]
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.[106]
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".[107] 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.[108] A number of parasites, including rhinonyssids and Trichomonas gallinae are known to be transferred between birds during episodes of billing.[109][110]
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 time,[111] and derives from the courtship of doves.[112]
Beak trimming
Because the beak is a sensitive organ with many sensory receptors, beak trimming (sometimes referred to as 'debeaking') is "acutely painful"
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.[114]
Amongst raptor keepers, this practice is commonly known as "coping".[115]
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
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 parrots, who are known for their dextrous extractive foraging techniques. Unlike probing foragers, the tactile pits in parrots are embedded in the hard keratin (or 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.[117]
See also
- Bird anatomy – Anatomy of birds
- Rostrum (anatomy) – Anatomy term
- Snout – Extended part of an animal's mouth
Footnotes
- ^ For an explanation of desmognathous, aegithognathous, etc. with images see "Catalogue of Species". 1891 – via Archive.org..
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