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 the platypus uses its bill to navigate underwater, detect food, and dig. the bill contains receptors that help detect prey. 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.(p20) 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 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 (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 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.(p83) 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'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 ratites, including the common ostrich, the 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 when billing, northern gannets raise their beaks high and clatter them against each other. 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 rhinonyssids and trichomonas gallinae 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 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 laying and broiler breeder flocks, because it helps reduce the damage the flocks inflict on themselves due to a number of stress-induced behaviors, including 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 raptor keepers, this practice is commonly known as "coping". bill tip organ kiwis have a probing bill that allows them to detect motion 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 ibises, shorebirds of the family scolopacidae, and kiwis. 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.