mouths.(p47) there is typically a septum 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.(p47) 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.(p375) 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.(p375) 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 falcons, have a small bony tubercule which projects from their nares. the function of this tubercule is unknown. some scientists suggest it may act as a baffle, slowing down or diffusing airflow into the nares (and thus allowing the bird to continue breathing without damaging its respiratory system) during high-speed dives, but this theory has not been proved experimentally. not all species that fly at high speeds have such tubercules, while some species which fly at low speeds do. operculum the rock dove's operculum is a mass at the base of the bill. the nares of some birds are covered by an operculum (plural opercula), a membraneous, horny or cartilaginous flap.(p117) in diving birds, the operculum keeps water out of the nasal cavity;(p117) 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,(p117) 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;(p84) though it is sometimes referred to as the cere, this is a different structure.(p151) tapaculos are the only birds known to have the ability to move their opercula.(p375) 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 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 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 (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 the nail is the black tip of this mute swan's beak. 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 objects. species which use strong grasping motions to secure their food (such as when catching and holding onto a large squirming frog) have very wide nails. certain types of mechanoreceptors, nerve cells that are sensitive to pressure, vibration, or touch, are located under the nail. the shape or color of the nail can sometimes be used to help distinguish between similar-looking species or between various ages of waterfowl. for example, the greater scaup has a wider black nail than does the very similar lesser scaup. juvenile "grey geese" have dark nails, while most adults have pale nails. the nail gave the wildfowl family one of its former names: "unguirostres" comes from the latin ungus, meaning "nail" and rostrum, meaning "beak". 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 this arctic tern chick still has its egg tooth, the small white projection near the tip of its upper mandible. 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.(p178) 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 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.(p427) 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.(p428) 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,(p427) while 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,(p178) though petrels keep theirs for nearly three weeks(p428) and marbled murrelets have theirs for up to a month. generally, the egg tooth drops off, though in songbirds it is reabsorbed.(p428) 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.: 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.: 63 more than a dozen types of carotenoids are responsible for the coloration of most red, orange, and yellow beaks.: 64 the hue of the color is determined by the precise mix of red and yellow pigments, while the 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.: 66 beak coloration helps to make displays using those beaks more obvious.(p155) in general, beak color depends on a combination of the bird's hormonal state and diet. colors are typically brightest as the breeding season approaches, and palest after breeding. 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. the presence and intensity of these peaks may indicate a bird's fitness, sexual maturity or pair bond status. 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 beaks of the now-extinct huia (female upper, male lower) show marked sexual 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 casques, and the female huia's slim, decurved bill was nearly twice as long as the male's straight, thicker one.(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. 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