Glossary of bird terms

The following is a glossary of common English language terms used in the description ofbirds—warm-blooded vertebrates of theclassAves and theonly living dinosaurs.^[1]Birds, who havefeathersand the ability tofly(except for the approximately 60 extant species offlightless birds), are toothless, havebeaked jaws,lay hard-shelledeggs, and have a highmetabolicrate, a four-chamberedheart,and a strong yet lightweightskeleton.

External anatomy (topography) of a typical bird: 1beak,2head,3iris,4pupil,5mantle,6lesser coverts,7scapulars,8coverts,9tertials,10rump,11primaries,12vent,13thigh,14tibio-tarsal articulation,15tarsus,16feet,17tibia,18belly,19flanks,20breast,21throat,22chin,23eyestripe

Among other details such as size, proportions and shape, terms defining bird features developed and are used to describe features unique to the class—especially evolutionary adaptations that developed to aidflight.There are, for example, numerous terms describing the complex structural makeup of feathers (e.g.,barbules,rachidesandvanes); types of feathers (e.g.,filoplume,pennaceousandplumulaceousfeathers); and their growth and loss (e.g.,colour morph,nuptial plumageandpterylosis).

There are thousands of terms that are unique to the study of birds. This glossary makes no attempt to cover them all, concentrating on terms that might be found across descriptions of multiple bird species by bird enthusiasts andornithologists.Though words that are not unique to birds are also covered, such as"back"or"belly,"they are defined in relation to other unique features of externalbird anatomy,sometimes called"topography."As a rule, this glossary does not contain individual entries on any of the approximately 11,000 recognized living individual bird species of the world.^[2]^[3]^[a]

A

addled eggs: Also,wind eggs;hypanema.^[5]Eggs that are not viable and will not hatch.^[6]See related:overbrooding.
afterfeather: Any structure projecting from the shaft of thefeatherat the rim of thesuperior umbilicus(at the base of thevanes), but typically a small area of downybarbsgrowing in rows or as tufts.^[b]Entirely absent in some birds—notably from many members of theColumbidaefamily (pigeons and doves)—afterfeathers can significantly increase the insulative attributes of a bird'splumage.^[8]
allopreening: A form ofsocial groomingamong birds, in which one birdpreensanother or a pair does so mutually. At times it may be used to redirect or sublimate aggression, such as one bird assuming a solicitation posture to indicate its non-aggression and invite allopreening by the aggressive individual.^[9]
alternate plumage: Also,nuptial plumage;breeding plumage.The plumage of birds during the courtship or breeding season. It results from aprealternate moultthat many birds undergo just prior to the season. The alternate plumage is commonly brighter than thebasic plumage,for the purposes of sexual display, but may also becryptic,to hide incubating birds that might be vulnerable on the nest.^[10]
altricial: Also defined:semi-altricial;altricial-precocial spectrum.Young that, at hatching, have their eyes closed; are naked or only sparsely covered indown feathers(psilopaedic); are not fully able to regulate their body temperature (ectothermic);^[11]and are unable to walk or leave the nest for an extended period of time to join their parents inforagingactivities (nidicolous), whom they rely on for food.^[12]The contrasting state isprecocialyoung, which are born more or less with their eyes open, covered in down,homeothermic,able to leave the nest and ambulate and to participate in foraging.^[11]^[13]The young of many bird species do not precisely fit into either the precocial or altricial category, having some aspects of each and thus fall somewhere on analtricial-precocial spectrum.^[14]A defined intermediate state is termedsemi-altricial,typified by young that, though born covered in down (ptilopaedic), are unable to leave the nest or walk and are reliant on their parents for food.^[15]^[16]
alula: Also,bastard wing;alular digit;alular quills.^[17]A small, freely-moving projection on theanterioredge of the wing of modernbirds(and a fewnon-avian dinosaurs)—a bird's "thumb" —the word is Latin and means 'winglet'; it is thediminutiveofala,meaning 'wing'. Alula typically bear three to five smallflight feathers,with the exact number depending on the species. The bastard wing normally lies flush against the anterior edge of the wing proper, but can be raised to function in similar manner to theslatsof airplane wings that aid inliftby allowing a higher than normalangle of attack.By manipulating the alula structure to create a gap between it and the rest of the wing, a bird can avoidstallingwhen flying at low speeds or when landing. Feathers on the alular digit are not generally considered to beflight feathersin the strict sense; though they are asymmetrical, they lack the length and stiffness of most true flight feathers. Nevertheless, alula feathers are a distinct aid to slow flight.^[18]
anisodactylous: Descriptive of tetradactyl (four-toed) birds in which the architecture of the foot consists of three toes projecting forward and one toe projecting backward (thehallux), such as in mostpasserinespecies.^[19]^[20]
anting: Also defined:passive anting.Aself-anointingbehaviour during which birds rub insects, usuallyantsand sometimesmillipedes,on theirfeathersand skin. Anting birds may pick up the insects in theirbilland rub them on their bodies, or may simply lie in an area with a high density of the insects and performdust bathing-like movements. Insects used for anting secrete chemical liquids such asformic acid,which can act asinsecticides,miticides,fungicidesandbactericides.The practice may also act to supplement a bird's ownpreen oil.A third purpose may be to render the insects more palatable, by causing removal of distasteful compounds. More than 200 species of bird are known to ant.^[21]"Passive anting" refers to when birds simply position themselves so as to allow insects to crawl through their plumage.^[6]
apical spot: A visible spot near the outer tip of a feather.^[22]
apterylae: Singular:apteryla.Also,apteria.Regions of a bird's skin, between thepterylae(feather tracts), which are free ofcontour feathers;filoplumesanddownmay grow in these areas.^[23]^[24]See related:pterylosis.
aviculture: The captive breeding and keeping of birds.^[6]
axilla: Also,axillar region;"underarm";"armpit".The "armpit" of a bird, often hostingcovert featherscalledaxillaries.^[25]
axillaries: Also,axillary feathers;lower humeral coverts;hypopteron.Covert feathersfound in theaxillar regionor "armpit" of a bird, which are typically long, stiff and white in colour.^[26]

B

back: The exterior region of a bird's upper parts between themantleand therump.^[27]
barb: Also defined:ramus(plural:rami).The individual structures growing out of theshaftthat collectively make up thevanesof the feathers, more or less interconnected by thehookletsof thebarbules,extending from each side of the distal part of the feather's shaft known as therachis.The central axis of a barb is known as the ramus.^[8]
barbules: Also,radius / radii;tertiary fibres.^[28]Also defined:proximal barbules;distal barbules;barbicels;hooklets(hamuli);pennulum;teeth.Just asbarbsbranch off on parallel sides of therachis,the barbs in turn have a set of structures called barbules, branching from each side of theramus.The base cells of the barbule form a plate from which a thinner stalk projects called the pennulum. At one more level of branching, the pennulum hosts small outgrowths from it called barbicels—which when found onpennaceous feathers,vary in structure depending on which side of a barb's ramus they project from. Proximal barbules (on the proximal side of the ramus) have ventral projections near the base called teeth, while growing from the pennulum are cilia—simple pointed structures. At the base of the proximal barbule, the "dorsal edge is recurved into aflange".^[29]The distal barbules (on the distal side of the ramus) have a thicker base with more elaborate teeth, and a longer pennulum with hooklets (also called hamuli^[30]) at the end, as well as cilia in greater number than on proximal barbules. The hooklets overlap one to four rows of proximal barbules on the next higher barb, locking into their flanges, thereby giving thevanestructure, strength, flexibility and stability.^[29]See also:friction barbules.
basic plumage: Also,winter plumage;non-breeding plumage.Also defined:supplementary plumage.The plumage of birds during the non-breeding season. It results from theprebasic moultthat many birds undergo just after the season, and sometimes (rarely) even a second non-breeding season moult (resulting in what's termed "supplementary plumage" ) prior to the next breeding season.^[c]The basic plumage is commonly duller than thealternate or nuptial plumage.^[10]^[31]
beak: Also,billorrostrum.An external anatomical structure of a bird's head, roughly corresponding with the "nose" of a mammal, that is used for eating,grooming,manipulating objects, killing prey, fighting, probing for food,courtshipand feeding young. Although beaks vary significantly in size and shape from species to species, their underlying structures follow a similar pattern. All beaks are composed of two jaws, generally known as theupper mandible(or maxilla) andlower mandible(or mandible),^[32]covered with a thin, horny sheath ofkeratincalled therhamphotheca,^[33]^[34]which can be subdivided into therhinothecaof the upper mandible and thegnathothecaof the lower mandible.^[35]Thetomia(singular: tomium) are the cutting edges of the two mandibles.^[36]Most species of birds have externalnares(nostrils) located somewhere on their beak—two holes—circular, oval or slit-like in shape—which lead to thenasal cavitieswithin the bird's skull, and thus to the rest of therespiratory system.^[37]Although the wordbeakwas, in the past, generally restricted to describing the sharpened bills ofbirds of prey,^[38]in modernornithology,the termsbeakandbillare generally used synonymously.^[33]
beak colour: The colour of a bird's beak results from concentrations ofpigments—primarilymelaninsandcarotenoids—in the epidermal layers, including therhamphotheca.^[39]In general, beak colour depends on a combination of the bird'shormonal stateanddiet.Colours are typically brightest as the breeding season approaches and palest after breeding.^[40]
beak trimming: Also,debeakingandcoping.The partial removal of thebeakofpoultry,especiallylayer hensandturkeys,although it may also be performed onquailandducks.Because the beak is a sensitive organ with many sensory receptors, beak trimming or debeaking is "acutely painful"^[41]to the birds it is performed on. It is nonetheless routinely done to intensively farmed bird species, because it helps reduce the damage the flocks inflict on themselves due to a number ofstress-induced behaviours, includingcannibalism,vent peckingandfeather 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, andneuromascan form along the cut edges. Food intake typically decreases for some period after the beak is trimmed. However, studies show that trimmed poultry'sadrenal glandsweigh less, and their plasmacorticosteronelevels are lower than those found in untrimmed poultry, indicating that they are less stressed overall.^[41]A less radical, separate practice, usually performed by anavian veterinarianor 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.^[42]"Coping" is the name for this practice amongstraptorkeepers.^[43]
belly: Also,abdomen.Thetopographicalregion of a bird'sunderpartsbetween the posterior end of the breast and thevent.^[44]
billing: Also,nebbing(chiefly UK).Describes the tendency of mated pairs of many bird species to touch or clasp each other'sbills.^[45]This behaviour appears to strengthenpair bonding.^[46]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.^[47]
bill tip organ: A region found near the tip of thebillin several types of birds that forage particularly by probing. The region has a high density of nerve endings known as thecorpuscles of Herbst.These consist of pits in the bill's surface lined with cells that sense pressure changes. The assumption is that these cells allow a bird to perform "remote touch", meaning that it can detect the movement of animals by pressure variations in water, without directly touching the prey. Bird species known to have bill-tip organs includeibises,shorebirds of the familyScolopacidaeandkiwis.^[48]
bird ringing: Also,bird banding.The attachment of small, individually numbered metal or plastic tags to the legs or wings of wild birds to enable individual identification. The practice helps in keeping track of bird movement and life history. Upon capture for ringing, it is common to take measurements and examine the conditions of feathermoult,amount of subcutaneous fat, age indications and sex. The subsequent recapture or recovery of banded birds can provide information on migration, longevity, mortality, population, territoriality, feeding behaviour and other aspects that are studied by ornithologists.Bird ringingis the term used in the UK and in some other parts of Europe, while the termbird bandingis more often used in the U.S. and Australia.^[49]
bird strike: The impact of a bird or birds with an airplane in flight.^[50]
body down: The layer of small, fluffydownfeathers that lie underneath the outercontour featherson a bird's body.^[51]Compare:natal downandpowder down.
breast: Thetopographicalregion of a bird's external anatomy between thethroatand thebelly.^[52]
breeding plumage: Seealternate plumage.
brood: The collective term for the offspring of birds, or the act ofbroodingthe eggs.^[53]
brooding: Seeegg incubation.
broodiness: Also defined:broody.The action or behavioural tendency of a bird to sit on aclutchof eggs toincubatethem, often requiring the non-expression of many other behaviours including feeding and drinking.^[54]The adjective "broody" is defined as "[b]eing in a state of readiness to brood eggs that is characterized by cessation of laying and by marked changes in behavior and physiology". Example usage: "a broody hen".^[55]
brood patch: A bare patch of skin that most female birds gain during thenesting seasonforthermoregulationpurposes, by shedding feathers close to the belly, in an area that will be in contact with the eggs duringincubation.The patch of bare skin is well supplied with blood vessels at the surface, facilitating heat transfer to the eggs.^[56]
brood parasite: Birds, such as thecommon goldeneye,indigobirds,whydahs,honeyguides,cowbirdsandNew World cuckoos,that lay their eggs in other birds' nests, in order to have their chicksincubatedthroughfledgingby the parents of another bird, often of another species.^[57]^[58]^[59]

C

calamus: Plural:calami.The basal part of thequillofpennaceous feathers,which embeds at its proximal tip in the skin of a bird. The calamus is hollow and has pith formed from the dry remains of the feather pulp. The calamus stretches between two openings—at its base is theinferior umbilicusand at its distal end is thesuperior umbilicus;therachisof the stem, hosting thevanes,continues above it.^[60]^[61]Calamus derives from the Latin for 'reed' or 'arrow'.^[62]
call: Specific types:alarm;contact;duet;antiphonal duetting;food begging;flight;mobbing.A type of bird vocalization tending to serve such functions as giving alarm or keeping members of aflockin contact—as opposed to a bird'ssong,which is longer, more complex and is usually associated withcourtship and mating.^[63]Individual birds may be sensitive enough to identify each other through their calls. Many birds that nest in colonies can locate their chicks using their calls.^[64]Alarm callsare used to sound alarm to other individuals.Food-begging callsare made by baby birds to beg for food, such as the "wah"of infantblue jays.^[65]Mobbing callssignalother individuals inmobbingspecies while harassing a predator. They differ from alarm calls, which alert other species members to allowescapefrom predators. As an example, thegreat tit,a European songbird, uses such a signal to call on nearby birds to harass a perchedbird of prey,such as an owl. This call occurs in the 4.5kHzrange,^[66]and carries over long distances. However, when such prey species are in flight, they employ an alarm signal in the 7–8 kHz range. This call is less effective at traveling great distances, but is much more difficult for both owls and hawks to hear (and detect the direction from which the call came).^[67]Contact callsare used by birds for the purpose of letting others of their species know their location.^[68]Relatedly,flight callsare vocalizations made by birds while flying, which often serve to keep flocks together.^[69]These calls are also used for when birds want to alert others that they are taking flight.^[70]Many birds engage induet calls—a call made by two birds at or nearly at the same time. In some cases, the duets are so perfectly timed as to appear almost as one call. This kind of calling is termedantiphonal duetting.^[71]Such duetting is noted in a wide range of families including quails,^[72]bushshrikes,^[73]babblerssuch asscimitar babblersand some owls^[74]and parrots.^[75]
canopy feeding: Also defined:double-wing feeding.Someherons,such as theblack heron,adopt an unusual position while hunting for prey. With their head held down in a hunting position, they sweep their wings forward to meet in front of their head, thereby forming an umbrella shaped canopy. To achieve full canopy closure, theprimariesandsecondariestouch the water, thenapefeathers are erected and the tail is drooped. The bird may take several strides in this position. One theory about the function of this behaviour is that it reducesglarefrom the water surface, allowing the bird to more easily locate and catch prey. Alternatively, the shade provided by the canopy may attract fish making their capture easier. Some herons adopt a similar behaviour calleddouble-wing feedingin which the wings are swept forward to create an area of shade, though a canopy is not formed.^[76]
carpal bar: A patch seen on theupperwingof some birds that usually appears as a long stripe or line. It is created by the contrast between the greater coverts and the other wing feathers.^[77]
caruncle: The collective term for the several fleshy protuberances on the heads and throats ofgallinaceousbirds, i.e.,combs,wattles,ear lobes and nodules. They can be present on the head, neck,throat,cheeksor around the eyes of some birds. Caruncles may be featherless, or present with a small array of scattered feathers. In some species, they may form pendulous structures oferectile tissue,such as the "snood"of thedomestic turkey.^[78]^[79]While caruncles are ornamental elements used by males to attract females to breed,^[80]it has been proposed that these organs are also associated with genes that encode resistance to disease,^[81]and for birds living in tropical regions, that caruncles also play a role in thermoregulation by making the blood cool faster when flowing through them.^[82]
casque: A horny ridge found on theupper mandibleof a bird's bill, especially used in relation tohornbillsandcassowaries,^[83]^[84]though other birds may have casques such ascommon moorhens,^[85]tufted puffins^[86]and (male)friarbirds.^[87]The ridge line on theupper maxillamay extend to a prominent crest on the front of the face and on the head, such as in the "flamboyant" crest of therhinoceros hornbill.^[88]Some hornbill casques contain a hollow space that may act as aresonance chamber,amplifyingcalls.^[89]Similar function has been proposed for cassowary casques, as well as for protection of the head while dense vegetation is traversed, as asexual ornamentand for use as a "shovel" for digging food.^[90]Compare:frontal shield.
cere: From the Latincerameaning 'wax', a waxy structure which covers the base of thebillsof some bird species from a handful of families—includingraptors,owls,skuas,parrots,turkeysandcurassows.The cere structure typically contains thenares,except in owls, where the nares are distal to the cere. Although it is sometimes feathered in parrots,^[91]the cere is typically bare and often brightly coloured.^[92]In raptors, the cere is a sexual signal which indicates the "quality" of a bird; the orangeness of aMontague's harrier's cere, for example, correlates to its body mass and physical condition.^[93]The colour or appearance of the cere can be used to distinguish between males and females in some species. For example, the malegreat curassowhas a yellow cere, which the female (and young males) lack,^[94]and the malebudgerigar's cere is blue, while the female's is pinkish or brown.^[95]
cheek: Also,malar / malar region.The area of the sides of a bird's head, behind and below the eyes.^[96]
chin: A small feathered area located just below the base of thebill'slower mandible.^[97]
cloaca: A multi-purpose opening terminating at theventat the posterior of a bird: birds expel waste from it; most birds mate byjoining cloaca(a "cloacal kiss"); and females lay eggs from it. Birds do not have aurinary bladderor external urethral opening and (with exception of theostrich) uric acid is excreted from the cloaca, along with faeces, as a semisolid waste.^[98]^[99]^[100]Additionally, in those few bird species in which males possess a penis (Palaeognathae[with the exception of thekiwis], theAnseriformes[with the exception ofscreamers] and in rudimentary forms inGalliformes^[101]^[102]) it is hidden within theproctodeumcompartment within the cloaca, just inside the vent.^[103]
cloacal kiss: Most male birds lack aphallusand instead have erectilegenital papillaat the terminus of theirvas deferens.When male and female birds of such species copulate, they each evert and then press together, or "kiss" their respectiveproctodeum(the lip of the cloaca). Upon the clocal kiss, the male's sperm spurts into the female's urodoeum (a compartment inside thecloaca), which then make their way into theoviduct.^[104]^[105]
clutch: All of the eggs produced by birds often at a single time in anest.Clutch size differs greatly between species, sometimes even within the samegenus.It may also differ intraspecies due to many factors includinghabitat,health, nutrition, predation pressures and time of year.^[106]Average clutch size ranges from one (as innorthern gannet^[107]) to about 17 (as ingrey partridge^[108]).
comb: Also,cockscomb(coxcomband other sp. variants).A fleshy growth or crest on the top of the head ofgallinaceousbirds, such asturkeys,pheasantsand domesticchickens.Its alternative name, cockscomb (or coxcomb) reflects that combs are generally larger on males than on females (a male gallinaceous bird is called acock). Comb shape varies considerably depending on the breed or species of bird. The "comb" most often refers to chickens in which the most common shape is the "single comb" of a rooster from breeds such as theleghorn.Other common comb types are the "rose comb" of, e.g., the eponymousrosecomb;the "pea comb" of, e.g., thebrahmaandaraucana;and others.^[109]
colony: Also defined:seabird colony;breeding colony;communal roost;heronries;rookery.A large congregation of individuals of one or more species of bird that nest or roost in proximity at a particular location. Many kinds of birds are known to congregate in groups of varying size; a congregation of nesting birds is called abreeding colony.A group of birds congregating for rest is called acommunal roost.Approximately 13% of all bird species nest colonially.^[110]Nesting colonies are very common among seabirds on cliffs and islands. Nearly 95% of seabirds are colonial,^[111]leading to the usage,seabird colony,sometimes called arookery.Many species ofternsnest in colonies on the ground.Herons,egrets,storksand other large waterfowl also nest communally in what are calledheronries.Colony nesting may be an evolutionary response to a shortage of safe nesting sites and abundance or unpredictable food sources which are far away from the nest sites.^[112]
colour morph: Seemorph.
commissure: Depending on usage, may refer to the junction of theupperandlowermandibles,^[113]or alternately, to the full-length apposition of the closed mandibles, from the corners of the mouth to the tip of thebeak.^[114]
contact call: A type ofcallused by birds for the purpose of letting others of their species know their location.^[68]
corpuscles of Herbst: Nerve-endings similar to thePacinian corpuscle,found in themucous membraneof thetongue,in pits on the beak and in other parts of the bodies of birds. They differ from Pacinian corpuscles in being smaller and more elongated, in having thinner and more closely placed capsules and in that the axis-cylinder in the central clear space is encircled by a continuous row ofnuclei.^[115]
coverts: Also,covert feathers;tectrices– singular:tectrix.A layer of non-flight feathers overlaying and protecting thequillsof flight feathers. At least one layer of covert feathers appear both above and beneath theflight feathersof the wings as well as above and below therectricesof the tail.^[116]These feathers may vary widely in size. For example, the upper tail tectrices of peacocks—the malepeafowl—rather than its rectrices, are what constitute its elaborate and colourful "train".^[117]There are a number of types and subtypes of covert feathers—primary, secondary, greater, lesser, marginal, median, etc.—see broadlywing covertsandtail coverts.
cranial kinesis: Also defined:prokinesis,amphikinesisanddistal rhynchokinesis.Movement of theupper mandiblein relation to the front of the skull. There is very little of this movement in birds that feed primarily through grazing and thus do not need to open their bills very widely. This is in contrast to parrots, which use their bills to manipulate food and as a support when climbing trees. There are multiple types of cranial kinesis:prokinesis,where the bill moves only at the craniofacial hinge;amphikinesis,where the whole upper jaw is raised; anddistal rhynchokinesis,where the bill flexes somewhere along the length of the bill, compared to just at the base.^[118]
crest feather: Collectively, the/acrest.Long crest feathers are sometimes calledquill feathers.^[119]Also defined:recumbent crestsandrecursive crests.A type ofsemiplumefeather with a longrachiswithbarbson either side, that often presents as a prominent tuft on thecrownand (or through) the neck and upperback.^[120]^[121]Birds with crests includeVictoria crowned pigeons,northern lapwings,macaroni penguinsand others, but the most recognizable arecockatoosandcockatiels,which can raise or lower their crests at will and use them to communicate with fellow members of their species, or as a form of defence to frighten away other species that approach too closely, making the bird appear larger when the crest is suddenly and unexpectedly raised.^[122]In some species the position of the crest is a threat signal that can be used to predict behaviour. InSteller's jays,for example, a raised crest indicates a likelihood of attack, and a lowered crest indicates a likelihood of retreat.^[123]Crests can berecumbentorrecursive,depending on the species. The recumbent crest, such as inwhite cockatoos,has feathers that are straight and lie down essentially flat on the head until fanned out.^[124]The recursive crest, such as insulphur-crested cockatoosandMajor Mitchell's cockatoos,is noticeable even when its feathers are not fanned out because they curve upward at the tips even when lying flat, and when standing up, often bend slightly forward toward the front of the head. Some birds, likegalahs(also known as the rose-breasted cockatoo), have modified crests that have both recumbent and recursive features.^[122]
crissum: The feathered area between theventand thetail.Also, the collective name for theundertail coverts.^[125]Thecrissal thrasherderives its name from the term, having distinctive colouring in the region, in contrast with the balance of itsplumage.Other birds having distinctive crissum colouration includeLe Conte's thrashersandgrey catbirds.^[126]
crop: An expanded, muscular pouch near thegulletor throat found in some but not all birds. It is a part of the digestive tract, essentially an enlarged part of theesophagus,used for the storage of food prior to digestion. As with most other organisms that have a crop, birds use it to temporarily store food. In adult doves and pigeons, the crop can producecrop milkto feed newly-hatched chicks.^[127]
crop milk: A secretion from the lining of thecropof parent birds that isregurgitatedto young birds. It is found among allpigeons and doveswhere it is referred to aspigeon milk.An analogue to crop milk is also secreted from the esophagus offlamingosand somepenguins.^[128]^[129]^[130]Crop milk bears little physical resemblance tomammalian milk,the former being a semi-solid substance somewhat like pale yellowcottage cheese.It is extremely high inproteinandfat,containing higher levels than cow or human milk^[131]and has been shown to also containantioxidantsand immune-enhancing factors.^[132]
crown: Also defined:occiput / hindhead.The portion of a bird's head found between theforehead—demarcated by an imaginary line drawn from the anterior corners of the eyes—and through the "remainder of the upper part of the head", to thesuperciliary line.Theocciputorhindhead,is the posterior part of the crown.^[133]
cryptic plumage: Also defined:phaneric plumage.Plumage of a bird that is camouflaging. For example, the white winter plumage ofptarmigansis cryptic as it serves to conceal it in snowy environments.^[134]The opposite, "advertising" plumage, is termed "phaneric", such as male birds in colourfulnuptial plumagefor sexual display, making them stand out to a high degree.^[135]
culmen: Thedorsalridge of theupper mandible.^[136]Likened by ornithologistElliott Couesto 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.^[137]Thebill's length along the culmen is one of the regular measurements made duringbird ringing^[138]and is particularly useful in feeding studies.^[139]The shape or colour of the culmen can also help with the identification of birds in the field. For example, the culmen of theparrot crossbillis strongly decurved, while that of the very similar appearingred crossbillis more moderately curved.^[140]

D

definitive plumage: Adultplumagesufficiently developed and fixed following the juvenile years such that it no longer changes significantly in appearance with age.^[31]
diastataxis: The state of lacking a fifthsecondary featheron each wing, which occurs in birds in more than 40 non-passerine families. In these birds, the fifth set of secondarycovert feathersdoes not cover anyremex.Loons,grebes,pelicans,hawksandeagles,cranes,sandpipers,gulls,parrotsandowlsare among the families missing this feather.^[18]
dietary classification terms (-vores): Birds may be classified by terms related to the types of foods they forage for and eat.^[141]The -vore suffix is derived from the Latinvorare,meaning 'to devour'. Equivalent adjectives can be formed through use of the suffix -vorous.^[142]For example,granivore(n.) /granivorous(adj.). Generally, classification terms are used based on predominance of food source and/or specialization. There can be much cross-over and mi xing between classifications. For example, insectivores and piscivores may at times be described more broadly as types of carnivores, and hummingbirds, though they do eat insects, are often described as nectarivores, rather than insectivores, as nectar is a specialized and predominant food foraging source for that bird family.^[143]The feeding strategies of birds is intimately tied to their physiology and evolutionary development. For example,beakshape and structure such as the makeup of thetomia,the presence (or not) of abill tip organand myriad other adaptations are tied to a species' feeding strategies.^[144]Feeding habits also correlate with aspects of brain development and size. For example, thespatial memoryof birds that store food in various locations has been shown to be highly developed and contributes towards success at that feeding tactic.^[145]
dimorphism: Seesexual dimorphism.
diving: Also defined:surface diving;plunge diving.Some birds dive into the water for food. Two diving strategies are differentiated:surface divingbirds dive from the surface of the water and swim actively underwater; andplunge divingbirds dive from the air into the water. Plunge diving birds may use the momentum from the plunge to propel themselves underwater, whereas others may swim actively.^[152]
down: Also,down feathersorplumulaceous feathers.The down of birds—their plumulaceous feathers, as opposed topennaceous feathers—are a layer of fine, silky feathers found under the tougher exterior feathers, that are often used by humans as a thermal insulator and padding in goods such as jackets, bedding, pillows and sleeping bags. Considered to be the "simplest" of all feather types,^[153]down feathers have a short or vestigialrachis(shaft), fewbarbsandbarbulesthat lack hooks,^[154]and (unlikecontour feathers) grow from both thepterylaeand theapteria.^[61]Very young birds are often only clad in down. The loose structure of down feathers traps air, which helps to insulate birds against heat loss^[51]and contributes to the buoyancy of waterbirds. Species that experience annual temperature fluctuations typically have more down feathers following their autumn moult.^[155]There are three types of down:natal down,body downandpowder down.^[156]
drumming: A form of non-vocal communication engaged in by members of thewoodpeckerfamily. It involves thebeakstriking a hard surface multiple times per second. The drumming pattern, the number of beats per roll and the gap between rolls is specific to each species. Drumming is usually associated withterritorial behaviour,with male birds drumming more frequently than females.^[157]Drummingmay also refer to thesoundsproduced by the specialised outer tail-feathers ofsnipein the course of their courtship display flights.^[158]

E

Eggsof: ostrich, emu, kiwi and chicken

ear-coverts: Smallcovertfeathers located behind a bird's eye, in one to four rows, which cover the ear opening (bird ears have no external features^[159]) and may aid in the acuity of bird hearing.^[160]
egg: Also defined:eggshell;yolk;albumen;chalaza.The organic vessel containing thezygote,in which birds develop until hatching. Eggs are usually oval in shape, and have a base white colour from the predominantcalcium carbonatemakeup of the outer shell, called theeggshell,thoughpasserinebirds especially may have eggs of other colours,^[161]such as through deposition ofbiliverdinand its zincchelate,which give a green or blue ground colour, andprotoporphyrinwhich produces reds and browns.^[162]A viable bird egg (as opposed to a non-viable egg:seeaddled eggs) consists of a number of structures. The eggshell is 95–97% calcium carbonate crystals, at least in chickens, stabilized by a protein matrix,^[163]^[164]^[165]without which the crystalline structure would be too brittle to keep its form; the organic matrix is thought to have a role in deposition of calcium during the mineralization process.^[166]^[167]^[168]The structure and composition of the avian eggshell serves to protect the egg against damage and microbial contamination, prevention of desiccation, regulation of gas and water exchange for the growing embryo and provides calcium forembryogenesis.^[164]Inside the eggshell are two shell membranes (inner and outer), and at the center is ayolk—a spherical structure, usually some shade of yellow, to which the fertilizedgameteattaches and which the embryonic bird uses as sustenance as it grows. The yolk is suspended in thealbumen(also calledegg whiteorglair / glaire) by one or two spiral bands of tissue called thechalazae.^[169]The albumen protects the yolk and provides additional nutrition for the embryo's growth,^[170]though it is made up of approximately 90% water in most birds.^[171]Prior to fertilization, the yolk is a singlecell ovumor egg cell; one of the few single cells that can be seen by the naked eye.^[172]
egg binding: An egg that while traversing the reproductive tract during the process of being laid, becomes stuck near to the opening of thecloacaor further inside theoviduct.^[173]The condition may be caused by obesity, nutritional imbalances such as calcium deficiency, environmental stress such as temperature changes, or malformed eggs.^[174]
egg incubation: Also,brooding.The general care of unhatched eggs by parent birds (more often by females but by birds of both sexes), especially by temperature regulation through sitting on them, crouching or squatting over them, covering them with their wings, providing shade, wetting eggs and related behaviours. The target temperature of most species is 37 °C (99 °F) to 38 °C (100 °F). In monogamous species incubation duties are often shared, whereas in polygamous species one parent is wholly responsible for incubation. Warmth from parents passes to the eggs throughbrood patches—areas of bare skin on the abdomen or breast of the incubating birds. Incubation can be an energetically demanding process; adultalbatrosses,for instance, lose as much as 83 grams (2.9 oz) of body weight per day of incubation.^[175]^[176]^[177]
egg tooth: A small, sharp,calcifiedprojection on the beak that full-term chicks of most bird species have, which they use to chip their way out of theiregg.^[178]This white spike is located near the tip of theupper mandiblein most species (e.g.,gulls);^[179]near the tip of thelower mandibleinstead in a minority of others, such asnorthern lapwings;^[179]with a few species, such asEurasian whimbrels,black-winged stiltsandsemipalmated sandpipers,^[179]having one on each mandible.^[180]Despite its name, the projection is not an actualtooth(as the similarly-named projections of somereptilesare); instead, it is part of theintegumentary system,as areclawsandscales.^[181]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.^[182]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.^[183]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.^[180]
emargination: A pronounced narrowing at some variable distance along the feather edges at the outermostprimariesof large soaring birds, particularly raptors. Whether these narrowings are callednotchesor emarginations' depends on the degree of their slope.^[18]An emargination is a gradual change, and can be found on either side of the feather. A notch is an abrupt change, and is only found on the wider trailing edge of the remige. The presence of notches and emarginations creates gaps at the wingtip; air is forced through these gaps, increasing the generation of lift.^[184]
eye-ring: Also defined:orbital ring.A visible ring of feathers around a bird's eye; the eye-ring is often paler than the surrounding feathers. By contrast, an orbital ring is bare skin ringing the eye. In some species, such aslittle ringed plover,the orbital ring may be quite conspicuous.^[97]
eyestripe: Also,eye line / eyeline.A visible stripe on the feathers of a bird's head, often darker than the surrounding feathers, running through the eye region.^[97]Comparesupercilium.

F

Feathervariations

feather: Epidermalgrowths that form the distinctive outer covering, orplumage,onbirds.They are considered the most complexintegumentarystructures found in vertebrates,^[185]^[186]and, indeed, a premier example of a complexevolutionary novelty.^[187]Feathers are among the characteristics that distinguish the extant birds from other living groups.^[188]Although feathers cover most parts of the body of birds, they arise only from certain well-definedtractson the skin. They aid in flight, thermal insulation and waterproofing, with their colouration helping in communication andprotection.^[189]Although there are many subdivisions of feathers, at the broadest levels, feathers are either classified as i)vaned feathers,which cover the exterior of the body and includepennaceous feathers,or ii)down feathers,which grow underneath the vaned feathers. A third rarer type of feather, thefiloplume,is hairlike and (if present in a bird; they are entirely absent inratites^[190]) grows alongside the contour feathers.^[185]A typical vaned feather features a main shaft called thequillwith an upper section called therachis.Fused to the rachis are a series of branches, orbarbs;the barbs in turn havebarbulesbranching off them, and they in turn branch yet again with a series of growths calledbarbicels,some of which have minute hooks calledhookletsfor cross-attachment. Down feathers are fluffy because they lack barbicels, so the barbules float free of each other, allowing the down to trap air and provide excellent thermal insulation. At the base of the feather, the rachis expands to form the hollow tubularcalamuswhich inserts into afolliclein theskin.The basal part of the calamus is without vanes. This part is embedded within the skin follicle and has an opening at the base (proximal umbilicus) and a small opening on the side (distal umbilicus).^[191]
feather pecking: A behavioural problem in which one bird repeatedly pecks at the feathers of another, that occurs most frequently amongstdomestic hensreared for egg production,^[192]^[193]although it is seen in otherpoultrysuch aspheasants,^[194]turkeys,^[195]ducks^[196]and sometimes in farmedostriches.^[197]Two levels of severity are recognised: "gentle" and "severe".^[198]
feather-plucking: Alsofeather-picking,feather damaging behaviourorpterotillomania.^[199]Amaladaptive,behavioural disorder commonly seen in captive birds which chew, bite or pluck their own feathers with their beak, resulting in damage to the feathers and occasionally the skin.^[200]^[201]It is especially common amongPsittaciformes,with an estimated 10% of captive parrots exhibiting the disorder.^[202]The chief areas of the body that are pecked or plucked are the more accessible regions such as the neck, chest,flank,inner thigh andventralwing area.Contouranddown feathersare generally identified as the main targets, although in some cases,tailandflight feathersare affected. Although feather-plucking shares characteristics withfeather pecking,commonly seen in commercial poultry, the two behaviours are currently considered to be distinct, as in the latter, the birds peck at and pull out the feathers ofother individuals.
feather tract: See:pterylae.
fecal sac: Also,faecal sac.A mucous membrane, generally white or clear with a dark end,^[203]that surrounds thefecesof some species ofnestlingbirds,^[204]and allows parent birds to more easily removefecal materialfrom thenest.The nestling usually produces a fecal sac within seconds of being fed; if not, a waiting adult may prod around the youngster'scloacato stimulateexcretion.^[205]Young birds of some species adopt specific postures or engage in specific behaviours to signal that they are producing fecal sacs.^[206]For example, nestlingcurve-billed thrashersraise their posteriors in the air, while youngcactus wrensshake their bodies.^[207]Other species deposit the sacs on the rim of the nest, where they are likely to be seen (and removed) by parent birds.^[206]Not all species generate fecal sacs. They are most prevalent inpasserinesand their near relatives, which have young that remain in the nest for longer periods.^[205]
filoplume: Also,filoplume feather;hair feather,thread feather.A hairlike type of feather that, if present in a bird (they are entirely absent inratites^[190]) grows alongside the contour feathers.^[185]The typical filoplume is silky in appearance, lacks pith and asuperior umbilicousopening, has a very slender, straight shaft lacking differentiation intocalamusandrachis,and is naked or has only a fewbarbs(that lack cross-attachment) at the distal end. They are closely associated withcontour feathersand often entirely hidden by them, with one or two filoplumes attached and sprouting from near the same point of the skin as each contour feather, at least on a bird's head, neck and trunk.^[208]^[209]Filoplume feathers host a cluster of sensory corpuscles at their base,^[210]that serve to detect air currents that affect contour and flight feathers.^[92]Filoplumes are one of the three major classes of feathers, the others beingpennaceousandplumulaceousfeathers.
flange: Also,recurved margin."The thickened dorsal edge of the bases of pennaceousbarbules,generally recurved in proximal barbules, and frequently so in distal barbules also "^[211]which anchor thehooklets.^[29](Seediagram;refer to figures 3 [in which the flange is referred to as the "folded edge" ] and 6 [in which the mechanism of interlocking between a flange andhookletis shown].)
flanks: Thetopographicalregion of theunderparts^[d]sketched "between the posterior half of the abdomen and therump".^[96]
fledge: Also,fledging.The stage in a young bird's life when the feathers and wing muscles are sufficiently developed for flight, or describing the act of a chick's parents in raising it to that time threshold.^[212]
fledgling: A juvenile bird during the period it is venturing from or has left thenestand is learning to run and fly; a young bird during the period immediately afterfledging,when it is still dependent upon parental care and feeding.^[213]
flight: Most birds canfly,which distinguishes them from almost all other vertebrate classes (cf.batsandpterosaurs). Flight is the primary means of locomotion for most bird species and is used for breeding, feeding and predator avoidance and escape. Birds have various adaptations for flight, including a lightweight skeleton, two large flight muscles, the pectoralis (which accounts for 15% of the total mass of the bird) and the supracoracoideus, as well as modified forelimbs (wings) that serve asaerofoils.^[214]Wing shape and size generally determine a bird species' type of flight; many birds combine powered, flapping flight with less energy-intensive soaring flight. About 60 extant bird species areflightless,as were many extinct birds.^[215]Flightlessness often arises in birds on isolated islands, probably due to limited resources and the absence of land predators.^[216]Though flightless, penguins use similar musculature and movements to "fly" through the water, as doauks,shearwatersanddippers.^[217]
flight feather: Also,Pennae volatus.^[218]The long, stiff, asymmetrically shaped, but symmetrically pairedpennaceous featherson the tail orwingsof a bird; those on the tail are calledrectrices(singular:rectrix), while those on the wings are calledremiges(singular:remex). Based on their location, remiges are subdivided intoprimaries,secondariesandtertiaries.^[219]
foot paddling: A foraging behaviour of gulls in which individuals stand at a location, often in shallow water, and perform rapid stepping actions that are thought to make subterranean worms or other food rise to the surface.^[220]
forehead: The portion of a bird's head extending "up and back from thebillto an imaginary line joining the anterior corners of the eyes ".^[133]
fovea: Plural:foveasorfoveae.A small cavity in theretinaof the eye that hosts a large number of light receptors; more than anywhere else on the retina. About one half of bird species with fovea have a single one, but uniquely in birds,^[221]some, such asterns,kingfishersandhummingbirds,have a second fovea,^[222]called the temporal fovea, that assists in judging speed and distance and increases visual acuity. Birds that do not have a second fovea will sometimes bob their head to improve their visual field.^[223]
friction barbules: A specialized type ofbarbulelocated on the distal part of the innervaneofprimary featherson the wings of most flighted birds. Friction barbules support lobe-shaped ( "lobular" )barbicelsthat are broader than the typical barbicel hosted by other vaned feathers, and which in turn support morehooklets.The theory is that the augmented surface area and other adaptations significantly increases grip throughfrictionwhen the outer web of barbs of one primary feather come into contact and rub against the inner web of barbs of another primary feather it overlays, thereby preventing slippage during the rigors of flight.^[224]^[225]Friction barbules are found only on those parts of primary feathers that are in "zones of overlap" with neighboring primaries. The theory (and the use of "friction" in the title of the defined phrase) has been criticized. InAvian Flight(2005), the author notes that "most birds open and close their wings during every wing beat cycle", and proposes that the energetic cost to overcome friction during the "wing extension and flexion" of each beat cycle would be prohibitive. Offered instead is the theory that the function of these specialized barbules is to lock the primary feathers together on the wings' downstroke, during which highpressurefrom below the wings' surface would otherwise tend to cause the feathers to spread.^[226]
frontal shield: Alsofacial shield;face shield;frontal plate.A hard or fleshy plate extending from the base of theupper mandibleover theforeheadof several bird species including somewater birdsin therail family,especially thegallinules and moorhens,swamphensandcoots,as well as injacana.^[227]While most face shields are made up of fatty tissues, some birds, such as certainturacos,e.g., thered-crested turaco,have face shields that are hard extensions of the mandible.^[228]The size, shape and colour may exhibittestosterone-dependent variation in either sex during the year.^[229]Functionality appears to relate to protection of the face while feeding in or moving through dense vegetation, as well as tocourtship displayandterritorialdefence.^[230]Compare:casque.
furcula: Also,wishbone;merry-thought.From the Latin for "little fork", the furcula is a forkedbone,also found in some dinosaurs, located below the neck and formed by the fusion of the twoclavicles.Its primary function is in the strengthening of thethoracicskeleton to withstand the rigors offlight.It works as a strut between a bird's shoulders, and articulates to each of a bird'sscapulae.In conjunction with thecoracoidand the scapula, it forms a unique structure called the triosseal canal, which houses a strong tendon that connects thesupracoracoideusmuscles to thehumerus.This system is responsible for lifting the wings during a recovery stroke. As the thorax is compressed by the flight muscles during a downstroke, the upper ends of the furcula spread apart, expanding by as much as 50% of its resting width, and then contract. Furcula may also aid inrespiration,by helping to pump air through theair sacs.^[231]^[232]

G

gape: The interior of the open mouth of a bird.^[233]The width of the gape can be a factor in the choice of food.^[234]
gape flange: The region where theupperandlowermandibles join together at the base of thebeak.^[233]When born, the chick's gape flanges are fleshy. As it grows into afledgling,the gape flanges remain somewhat swollen and can thus be used to recognize that a particular bird is young.^[235]Gape flanges can serve as a target for food for parents, and when touched, stimulate the nestling to open its mouth to eat.^[236]
gizzard: Also,ventriculus;gastric mill;gigerium.A specializedstomachorgan found in the digestive tract of some birds constructed of thick muscular walls that is used for grinding up food, often aided by particles of stone or grit. Food, after going through thecropandproventriculus,passes into the gizzard where it can be ground with previously swallowed stones and passed back to the proventriculus, and vice versa. Bird gizzards are lined with a tough layer made of a carbohydrate-protein complex called koilin, that protects the muscles in the gizzard.^[237]^[238]
gleaning: Specialized cases:foliage gleaning;hover-gleaning;crevice-gleaning.The strategy of gleaning over surfaces by birds to catch invertebrate prey—chieflyinsectsand otherarthropods—by plucking them from foliage or the ground, from crevices such as of rock faces and under the eaves of houses, or even, as in the case of ticks and lice, from living animals. Gleaning the leaves and branches of trees and shrubs is called "foliage gleaning", which can involve a variety of styles and maneuvers. Some birds, such as thecommon chiffchaff^[239]ofEurasiaand theWilson's warblerofNorth America,feed actively and appear energetic. Some will even hover in the air near a twig while gleaning from it; this behaviour is called "hover-gleaning". Other birds are more methodical in their approach to gleaning, even seeming lethargic as they perch upon and deliberately pick over foliage. This behaviour is characteristic of thebay-breasted warbler^[240]and manyvireos.Another tactic is to hang upside-down from the tips of branches to glean the undersides of leaves. Tits such as the familiarblack-capped chickadeeare often observed feeding in this manner. Some birds, like theruby-crowned kinglet,use a combination of these tactics. "Crevice-gleaning" is a niche particular to dry and rocky habitats. Gleaning birds are typically small with compact bodies and have small, sharply pointedbeaks.Birds often specialize in a particular niche, such as a particular stratum of forest or type of vegetation.^[241]
gnathotheca: Therhamphotheca(thin horny sheath ofkeratin) covering thelower mandible.^[33]^[34]^[35]
gonys: Theventralridge of thelower mandible,created by the junction of the bone's two rami, or lateral plates.^[242]
gonydeal angle: Also,gonydeal expansion.The proximal end of the junction created by thegonystwo rami, or lateral plates—the place where the two plates separate. The size and shape of the gonydeal angle can be useful in identifying between otherwise similar species.^[40]
gonydeal spot: The spot near the gonydeal expansion, in adults of many bird species but especially in gulls, usually reddish or orangish in colour, that triggersbegging behaviour.Gull chicks, for example, peck at the spot on its parent's bill, which in turn stimulates the parent toregurgitatefood.^[40]^[243]
gorget: A patch of colouredfeathersfound on the throat or upper breast of some species ofbirds.^[242]It is a feature found on many malehummingbirds,particularly those found in North America, whose gorgets are typicallyiridescent.^[244]Other species having gorgets include thepurple-throated fruitcrow^[245]and thechukar partridge.^[246]The term is derived from thegorgetused in military armor to protect the throat.^[247]
grooming: See entry forpreening.
gular region: The posterior part of the underside of a bird's head, described as "a continuation of the chin to an imaginary line drawn between theangles of the jaw".^[8]See also:gular skin.
gular skin: Also defined,gular sac / throat sac;gular pouch;gular flutter.Describes thegular regionwhen it is featherless. In many species, the gular skin forms a flap, orgular pouch,which is generally used to store fish and other prey while hunting. In many others, such asfrigatebirds,the gular skin may overlie asac—thegular sacorthroat sac—that may be dramatically inflated by males during courtship display. In some species the gular region is used for thermoregulation, by the fluttering of thehyoid boneand surrounding muscles, vibrating as many as 735 times per minute, which causes an increase in heat dissipation from the gular skin.^[248]^[249]^[250]

H

hallux: Also,hind toe;first digit.Specialized types:incumbentandelevated.A bird's usually rear-facing toe; itshind toe.When it is attached near the base of themetatarsusit is termedincumbent,and when projecting from a higher portion of the metatarsus (as inrails), it is termedelevated.The hallux is a bird'sfirst digit,in many birds being the sole rear-facing toe (including in mostpasserinespecies that haveanisodactylousfeet) and is homologous to the humanbig toe.^[251]^[252]^[253]
home range: Also defined:territory.The area of land in which a bird performs most of its activities. When the area is guarded (in whole or in part) from individuals of the same species it is called aterritory.^[254]These territories may be temporary or permanent, breeding or non-breeding. Breeding territories may be of four types: i) an all-purpose territory where all activities are conducted; ii) a separate breeding and feeding territory (or just a breeding territory, with foraging conducted outside of it); iii) a territory surrounding the nest and a very small area outside of it; iv) a small territory used solely for display within alek.^[255]The size of the home range can be affected by what food a bird eats and how much the bird weighs. Birds that weigh more usually have larger ranges, andcarnivoreson average have larger ranges than non-carnivores.^[256]
Humphrey–Parkes terminology: A system ofnomenclaturefor theplumageof birds proposed in 1959 by Philip S. Humphrey and Kenneth C. Parkes^[10]to make the terminology for describing bird plumages more uniform.^[257]Examples of Humphrey–Parkes terminology versus traditional terminology may be seen in the entries forprealternate moultandprebasic moult.
hyoid apparatus: The system of bones to which the tongue is attached. It usually includes the tongue bone, to which the tongue is actually attached,^[258]the basihyal, behind the tongue bone, the urohyal, itself behind the basihyal, a pair of ceratobranchial bones, and a pair of epibranchial.^[259]The latter two bones form the hyoid horns, which are contained in a pair of fascia vaginalis. This allows the tongue to slide out smoothly. The hyoid apparatus is attached to the larynx.^[258]

I

in pin: See#pin feather
inferior umbilicus: Also,proximal umbilicus.A small opening located at the bottom tip of a feather's shaft, i.e., at the base of thecalamus,that is embedded within the skin of a bird. The growth of the feather is fed by the flow through the inferior umbilicus of a nutrient (ornutritive) pulp of highly vasculardermalcells (sometimes called thenutritive dermis;a part of what is termed theMalpighian layer), that in turn produce a nourishingplasma.In mature feathers the opening is sometimes sealed over by akeratinousplate.^[189]^[260]^[261]Compare:superior umbilicus.
inner wing: Also defined:outer wing.Theinner wingof a bird is that portion of the wing stretching from its connection to the body and through the "wrist" joint. Theouter wingstretches from the wrist to the wingtip.^[262]
iris: The coloured outer ring that surrounds a bird'spupil.Though brown predominates, the iris may be of or include a variety of colours—red, yellow, grey, blue, etc.—and the colouration may vary according to the age, sex and species.^[159]

J

jizz: Also,gestalt,^[263]of which the term may be a corruption^[264](or possibly deriving from theair forceacronym GISS for "General Impression of Size and Shape (of an aircraft)", associated withWorld War IIlingo; though this derivation may by anachronistic asjizzwas first used in birdwatching as early as 1922),^[265]it describes the overall impression or appearance of abird— "the indefinable quality of a particular species, the 'vibe' it gives off"^[266]—garnered from such features as shape, posture, flying style or other habitual movements, size and colouration combined with voice, habitat and location.^[267]^[268]^[269]Example use:"It had the jizz of a thrush, but I couldn't make out what kind."

K

keel: Also,carina.An extension of thesternum(breastbone) which runs axially along the midline of the sternum and extends outward, perpendicular to the plane of theribs.The keel provides an anchor to which a bird's wing muscles attach, thereby providing adequateleverageforflight.Keels do not exist on all birds; in particular, someflightless birdslack a keel structure. Historically, the presence or absence of a pronounced keel structure was used as a broad classification of birds into two orders:Carinatae(from Latincarina,'keel'), having a pronounced keel; andratites(from Latinratis,'raft'—referring to the flatness of the sternum), having a subtle keel structure or lacking one entirely. However, this classification has fallen into disuse as evolutionary studies have shown that many flightless birds have evolved from flighted birds.^[270]^[271]
kleptoparasitism: A bird's propensity to steal food from other birds, either opportunistically, or on a regular basis.^[272]

L

lateral throat-stripe: A usually dark stripe of colour bordering the throat below both themoustachial stripeand thesubmoustachial stripe,if any.^[27]
lek: Also defined:lekking;dispersed lek.An aggregation of male birds gathered to engage in competitive displays (known aslekking) that may entice visiting females that are assessing prospective partners for copulation.^[273]These males are usually in sight of each other, but when they are only in earshot, it is called adispersed lekor anexploded lek.Mating usually occurs on the display area.^[274]
lore: Adj. form:loreal.Thetopographicalregion of a bird's head between the eye and thebeakon the sides of the head.^[96]
lower mandible: Also,mandible.The lower part of a bird'sbillor beak, roughly corresponding to the lower jaw of mammals, it is supported by a bone known as the inferior maxillary bone—a compound bone composed of two distinct ossified pieces. These ossified plates (orrami), which can be U-shaped or V-shaped,^[32]join distally (the exact location of the joint depends on the species) but are separatedproximally,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.^[34]The muscles that depress the lower mandible are usually weak, except in a few birds such as the starlings (and the extinctHuia), which have well-developeddigastric musclesthat aid in foraging by prying or gaping actions.^[275]In most birds, these muscles are relatively small as compared to the jaw muscles of similarly sized mammals.^[276]The outer surface is covered in thin horny sheath ofkeratincalledrhamphotheca,^[33]^[34]specially calledgnathothecain the lower mandible.^[35]Compare:upper mandible.

M

maintenance behaviour: Also,comfort behaviour.The name given to any behaviour or activity which a bird uses to maintain its plumage and soft parts:preening,bathing,anting,sunbathing, stretching, scratching, and other such activities.^[277]^[278]Roosting is also often considered to be a comfort behaviour.^[279]The phrase is sometimes used more inclusively, to describealllife-sustaining activities, such as feeding, drinking, predator avoidance, and locomotion,^[280]and even involuntary biological processes, likemoulting.^[281]
mandible: In birds, the wordmandible,alone, usually refers to thelower mandible.
mantle: The forward area of a bird's upper side sandwiched between thehindneckand the start of theback.However, ingullsandterns,the term is often used to refer to much of the upper surface below the hindneck.^[27]
migration: Also defined:partial migration.The regular seasonal movement, often north and south, undertaken by many species of birds. Bird movements include those made in response to changes in food availability, habitat, or weather. Sometimes journeys are not termed "true migration" because they are irregular (nomadism, invasions, irruptions) or only in one direction (dispersal, movement of young away from natal areas). Migration is marked by its annual seasonality.^[282]^[283]Non-migratory birds are said to beresidentor sedentary. Approximately 1,800 of the world's bird species are long-distance migrants.^[284]^[285]Many bird populations migrate long distances along a flyway. The most common pattern involves flying north in the northern spring to breed in the temperate orArcticsummer, and returning in the autumn to wintering grounds in warmer regions to the south. In the southern hemisphere the directions are reversed, but there is less land area in the far south to support long-distance migration.^[286]Not all populations within a species may be migratory; this is known as "partial migration". Partial migration is very common in birds of the southern continents. For example, in Australia, 44% of non-passerine birds and 32% ofpasserinespecies are partially migratory.^[287]Many, if not most birds migrate in flocks. For larger birds, flying in flocks reduces the energy cost, e.g.,geesein a V-formation may conserve 12–20% of the energy they would otherwise need if flying alone.^[288]^[289]
morph: Also,colour/color morph.Polymorphicvariance in the colouring of the plumage between individuals of the same species, unrelated to age, sex or season. For example, thesnow goosehas two plumage morphs, white (snow) or grey/blue (blue), thus the common description of individuals as either "snows" or "blues". At one time this colour morph resulted in the "blue goose" being classified as a separate species.^[290]^[291]
moult: Also,molt(chiefly U.S.) andmoulting / molting.The periodic replacement offeathersby shedding old feathers while producing new ones. Feathers are dead structures at maturity which are gradually abraded and need to be replaced. Adult birds moult at least once a year, although many moult twice and a few three times each year. It is generally a slow process, as birds rarely shed all their feathers at any one time; the bird must retain sufficient feathers to regulate itsbody temperatureand repel moisture. The number and area of feathers that are shed varies. In some moulting periods, a bird may renew only the feathers on the head and body, shedding the wing and tail feathers during a later moulting period. Some species of bird become flightless during an annual "wing moult" and must seek a protected habitat with a reliable food supply during that time. Typically, a bird begins to shed some old feathers, thenpin feathersgrow in to replace the old feathers. As the pin feathers become full feathers, other feathers are shed. This is a cyclical process that occurs in many phases. It is usuallysymmetrical,with feather loss equal on each side of the body. Because feathers make up 4–12% of a bird's body weight, it takes a large amount of energy to replace them. For this reason, moults often occur immediately after the breeding season, but while food is still abundant. The plumage produced during this time is calledpostnuptial plumage.^[292]
moult strategy: Specialized types defined:simple basic strategy;simple alternate strategy;complex basic strategy;complex alternate strategy.The pattern ofmoultsthat occurs regularly based on the season and time. There are four main moulting strategies: i) simple basic strategy, in which there is one moult per year, every year; ii) simple alternate strategy, in which there are two moults per year, every year; iii) complex basic strategy, in which there is one moult per year, except in the first year of life, where there is an additional moult; and iv) complex alternate strategy, in which there are two moults per year, except in the first year of life, where there is an additional moult.^[293]
moustachial stripe: Also,mustache stripe;whisker stripe;malar stripe.A dark feather colouration line running "obliquely from thegapedown and along the lower border of theear-coverts".Compare:submoustachial stripeandlateral throat-stripe.^[27]Birds with moustachial stripes include some of thepipitsas well asbuntings.^[27]

N

nail: A plate of hard horny tissue at the tip of the beak of all birds in the familyAnatidae(ducks, geese and swans).^[294]Thisshield-shaped structure, which sometimes spans the entire width of the beak, is often bent at the tip to form a hook.^[295]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,^[296]whilediving ducksuse theirs to prymolluscsfrom rocks.^[297]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.^[298]Certain types ofmechanoreceptors,nerve cells that are sensitive to pressure, vibration or touch, are located under the nail.^[299]
nape: Also defined:hindneck.The back of the head, behind thecrown.The area between the nape and themantleis known as the hindneck.^[27]
nares: The two holes—circular, oval or slit-like in shape—which lead to thenasal cavitieswithin the bird's skull, and thus to the rest of therespiratory system.^[37]In most bird species, the nares are located in the basal third of the upper mandible.Kiwisare a notable exception; their nares are located at the tip of their bills.^[92]A handful of species have no external nares.Cormorantsanddartershave primitive external nares as nestlings, but these close soon after the birdsfledge;adults of these species (andgannetsandboobiesof all ages, which also lack external nostrils) breathe through their mouths. There is typically aseptummade 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, includinggrouseandptarmigans,crowsand somewoodpeckers.^[37]
nasal canthus: The area where the eyelids come together at the anterior corner of the eye, on the side of thenares,as opposed to thetemporal canthus.^[133]
natal down: Also,neossoptiles.Also defined:protoptiles;mesoptiles.The layer ofdownfeathers that cover most birds at some point in their early development.Precocialnestlings are already covered with a layer of down when they hatch, whilealtricialnestlings develop their down layer within days or weeks of hatching.Megapodehatchlings are the sole exception; they are already covered withcontour featherswhen they hatch.^[300]The natal down coat is usually lost within a week or two of hatching.^[301]There are two different kinds of natal down feathers; protoptiles and mesoptiles.^[302]The former appears first^[303]and the latter appears second.^[304]Hathchlings born naked or with a diffuse coat of scant natal down feathers are calledpsilopaedic,while those born covered with a dense fuzz of natal down are termedptilopaedic.^[301]Compare:body downandpowder down.
nest: Also,bird nest.Specialized types:burrow;cavity;cup(adherent cup;statant cup);dome;dormitory;eyries(oraeries);hanging;ledge;mound;pendant;platform;roost(orwinter-nest);scrape;saucerorplate;andsphere.The spot in which a bird lays andincubatesitseggsand raises its young. Although the term popularly refers to a specific structure made by the bird itself—such as the grassy cup nest of theAmerican robinorEurasian blackbird,or the elaborately woven hanging nest of theMontezuma oropendolaor thevillage weaver—for some species, a nest is simply a shallow depression made in sand; for others, it is the knot-hole left by a broken branch, a burrow dug into the ground, a chamber drilled into a tree, an enormous rotting pile of vegetation and earth, a shelf made of dried saliva or a mud dome with an entrance tunnel. The smallest bird nests are those of somehummingbirds,tiny cups which can be a mere 2 cm (0.79 in) across and 2–3 cm (0.79–1.18 in) high.^[305]At the other extreme, some nest mounds built by thedusky scrubfowlmeasure more than 11 m (36 ft) in diameter and stand nearly 5 m (16 ft) tall.^[306]Not all bird species build nests. Some species lay their eggs directly on the ground or rocky ledges, whilebrood parasiteslay theirs in the nests of other birds, letting unwitting "foster parents" do all the work of rearing the young. Although nests are primarily used for breeding, they may also be reused in the non-breeding season for roosting and some species build special dormitory nests or roost nests (or winter-nest) that are used only for roosting.^[307]Most birds build a new nest each year, though some refurbish their old nests.^[308]The large eyries (or aeries) of some eagles are platform nests that have been used and refurbished for several years. In most species, the female does most or all of the nest construction, though the male often helps.^[309]The nest may form a part of the courtship display, such as inweaver birds.^[274]
nictitating membrane: Also,third eyelid.A transparent ortranslucentthirdeyelidthat is drawn across the eye for protection, to moisten it while maintaining vision.^[310]The nictitating membrane also covers the eye and acts as acontact lensin many aquatic birds.^[311]With the exception of pigeons and a few other species, most birds blink only with their nictitating membrane, and when not sleeping, use the eyelids chiefly only when the eye is threatened with some foreign matter.^[312]
nidicolous: Young that remain in their nest after hatching for an extended period of time and thus must befedby their parents.^[16]Contrast:nidifugous.
nidifugous: Young that leave the nest soon after hatching, joining their parents inforaging.^[16]Contrast:nidicolous.
non-breeding plumage: Seebasic plumage.
notch: A pronounced narrowing at some variable distance along the feather edges at the outermostprimariesof large soaring birds, particularly raptors. Whether these narrowings are called notches oremarginationsdepends on the degree of their slope.^[18]An emargination is a gradual change, and can be found on either side of the feather. A notch is an abrupt change, and is only found on the wider trailing edge of the remiges. The presence of notches and emarginations creates gaps at the wingtip; air is forced through these gaps, increasing the generation of lift.^[184]
nuptial plumage: Seealternate plumage.

O

oology: The scientific study of eggs.^[313]
operculum: Plural:opercula.A membraneous, horny orcartilaginousflap covering thenaresof some birds.^[314]^[315]For example, in diving birds, the operculum keeps water out of the nasal cavity;^[314]when the birds dive, the impact force of the water closes the operculum.^[316]
overbrooding: The not uncommon phenomenon of birds continuing tobroodeggs that are not viable and will not hatch, sometimes for lengthy periods of time beyond the normal incubation period.^[317]

P

Topographyof a typicalpasserine

passerine: Also,perching bird.Any bird of theorderPasseriformes, which includes more than half of all bird species. A notable feature of passerines compared to other orders of Aves is the arrangement of their toes—three pointing forward and one back—which facilitates perching. Sometimes known as perching birds or, less accurately, assongbirds,the passerines form one of the most diverse terrestrialvertebrateorders, with over 5,000 identifiedspecies.^[318]The order has roughly twice as many species asRodentia,the largest order ofmammals.There are more than 110 families of passerine birds, the second-most of anytetrapods(afterSquamata,the scaled reptiles).
pectinate claw: Also,feather comb.^[251]A claw on the middle toe of some birds, such as nightjars, herons, and barn owls,^[319]with a serrated edge. The degree ofserrationvaries from fine to coarse, depending on the species. It is used inpreening,functioning similar to a comb. Some birds may use it to straighten therictal bristles.^[320]It may also play an important role in parasite removal. For example, a study of barn owls found a significant correlation between the number of serrated teeth on an individual's pectinate claw, and the prevalence of itsliceinfestation.^[321]
pectoral tuft: Elongatedfeathers,often brightly coloured, which arise from the sides of thechestof some species of birds, including manysunbirds,spiderhuntersandflowerpeckers.^[322]They are typically hidden when the wing is folded on a perched bird, but prominent during courtship and territorialdisplays.^[323]In sunbirds and spiderhunters, these tufts may be yellow, red or orange; in flowerpeckers, they are usually white.^[322]The size of the tufts in some species can be an indicator of a male'sfitnessand status; males with larger tufts defend larger territories and have higher reproductive success.^[324]
pennaceous feather: Also,contour feather.A type offeatherpresent in most modern birds and in some other species ofmaniraptoriformdinosaurs.^[325]Pennaceous feathers have a stalk called aquill,with the basal part called acalamus,that is embedded in the skin exclusively atpterylae(feather tracts).^[61]The calamus is hollow and has pith formed from the dry remains of the feather pulp. The calamus stretches between two openings—at its base is theinferior umbilicusand at its distal end is thesuperior umbilicus;therachisof the main stem, hosting thevanes,continues above it.^[60]The rachides of contour feathers have an umbilical groove on their underside, with the vanes orvexilla,spreading to either side. The vanes comprise many flattenedbarbs,that are connected to one another byhookletsthat are anchored into flanges onproximal barbuleson neighboring barbs. Some specialized feathers are considered "modified" contour feathers, such asear covertsand bird eyelashes calledrictal bristles.^[61]Contrast:plumulaceous (down) feathersandfiloplumes.
philopatry: The habit of a bird returning to the area it first bred.^[326]
pigeon milk: The term forcrop milkwhen found among allpigeons and doves.^[129]
pileum: As defined in the treatiseManual of Ornithology: Avian Structure and Function,the "entiretop of the head, including theforehead,crownand occipital regions. "^[121]
pin feather: Also,blood feather.A developingfeatherencased in afeather sheaththat, unlike a fully developed feather, has a blood supply flowing through it. As such, damage to pin feathers can cause significant bleeding. Pin feathers may be the first feather growth during a bird's infancy, or those growing duringmoultingat any stage of a bird's life. As pin feather growth progresses, the blood supply is concentrated only in the base of thecalamus.Pin feathers begin to develop after the feather budinvaginatesa cylinder ofepidermal tissuearound the base of the dermal papilla, forming the feather follicle. At the base of the feather follicle, epithelial cells proliferate to grow the epidermal collar or cylinder. As the epidermal cylinder extends throughdermis,it differentiates into a protective peripheral sheath, longitudinal barb ridges andgrowth plates.Over time these barb ridges grow in ahelicalmanner, branching to createbarbsandbarbules,and fusing to form the central feather shaft. Moreover, the barb plate further differentiates intohookletsandcilia,while the marginal and axial plate die to form the intervening space within the feather structure.^[327]^[328]^[329]
pinioning: Also defined:pinion joint.The act of surgically removing onepinion joint—the joint of a bird's wing farthest from the body—thereby stopping the growth of theprimary feathers,to prevent flight. Theanimal welfareimpact of pinioning is subject to increasing debate. For example, it is known that the operation, which is often performed without pain relief, is just as painful in young birds as in mature birds, if not more so.^[330]Evidence also suggests that pinioning may cause aphantom limbsyndrome similar to what is observed in humanamputees.^[330]
pinions: The outermostprimaries—those connected to thephalanges.^[331]
plumage: Plumage (Latin:plūma'feather') refers both to the layer offeathersthat cover a bird and the pattern, colour and arrangement of those feathers.^[332]The pattern and colours of plumage differ between species and subspecies, often vary with age and may vary sharply between males and females of the same species that exhibitsexual dimorphism.Within species there can be differentcolour morphs.Despite the great variance, as broad observations: 1) among many bird species adult males are more brightly coloured than are females, especially when inalternate or nuptial plumage,as opposed tobasic or winter plumage;2) when male birds undergo theprebasic moultand attain their non-breeding plumage, they much more closely resemble females; and 3) the plumage of juvenile birds (of both sexes) tends to be relatively dull and inconspicuous and typically resembles that of adult females.^[333]
plumology: Also,plumage science.The name for the science that is associated with the study offeathers.^[334]^[335]
podotheca: Also,foot sheath.Also defined:boot;reticulate;scutellate;booted.The skin that covers the bare feet and legs. It usually consists of many small scales, calledscutes.This type of podotheca is described as being scutellate. When there are no scutes, and the podotheca is a smooth sheath, it is called a boot, and those with podotheca like this are called booted. The podotheca may also be reticulated with small and irregularly raised plates. This arrangement is considered to be reticulate.^[336]
powder down: Also,pulviplumes;feather dust.A special type ofdownthat occurs in a few groups of apparently unrelated birds. In some species, the tips of thebarbuleson powder down feathers disintegrate, forming fine particles ofkeratin,which appear as a powder, orfeather dust,among the feathers. These feathers grow continuously and are not moulted.^[337]In other species, the powder grains come from cells that surround the barbules of growing feathers.^[338]These specialized feathers are typically scattered among ordinary down feathers, though in some species, they occur in clusters.^[154]Allparrotshave powder down, with some species (including themealy parrot) producing copious amounts.^[339]It is also found intinamousandherons.^[154]The dust produced from powder down feathers is a knownallergenin humans.^[340]Compare:body down&natal down.
postjuvenal moult: Seeprebasic moult.
postnuptial moult: Seeprebasic moult.
prealternate moult: Also,prenuptial moult(sp. variation: in the U.S., sometimes molt).Themoultbefore the breeding season that most birds undergo, during which parts of thebasic plumageare shed andnuptial plumageis grown. "Prealternate moult" is used inHumphrey–Parkes terminology,and "prenuptial moult" is the traditional term. In either system of nomenclature, in juveniles this type of moult is numbered, 1st, 2nd, 3rd (etc.), until thedefinitive plumageis attained, after which the numbering is dropped.^[341]Unlike theprebasic moult,in the prealternate moult the majorflight feathersandprimary covertsare not replaced in most species, and the feathers involved typically are "only head and body feathers,wing covertsand sometimestertialsandrectrices".^[342]
prebasic moult: Also,postnuptial moult(sp. variation: in the U.S., sometimes molt).Also defined:postjuvenal moult.Themoultafter the breeding season that most birds undergo, during whichnuptial plumageis shed andbasic or winter plumageis grown. InHumphrey–Parkes terminology,prebasic moults in juveniles, prior to attainingdefinitive plumage,are numbered the 1st, 2nd, 3rd (etc.) prebasic moult. In traditional terminology, the first is called the postjuvenal moult, and thereafter the 1st, 2nd, 3rd (etc.) postnuptial moult. In both nomenclature systems, after definitive plumage is reached, the numbering is dropped.^[341]
precocial: Also defined:semi-precocial;altricial-precocial spectrum.Young that, at hatching, have their eyes open; are covered indown feathers(ptilopaedic); arehomeothermic;^[11]and are able to leave the nest soon after hatching and join their parents inforagingactivities (nidifugous). The contrasting state isaltricialyoung, which are born "helpless" —more or less naked, blind, ectothermic and unable to leave the nest.^[13]The young of many bird species do not precisely fit into either the precocial or altricial category, having some aspects of each and thus fall somewhere on analtricial-precocial spectrum.^[14]A defined intermediate state is termedsemi-precocial,typified by young born covered in down and with open eyes; that are usually able to walk shortly after hatching; but which remain mostly confined to the nest, relying on their parents for food (nidicolous).^[16]^[343]
preening: Also,grooming.Feathersrequire maintenance and birds preen or groom them daily, spending an average of around 9% of their daily time on this activity.^[344]Thebillis used to brush away foreign particles and to applywaxysecretions from theuropygial gland;these secretions protect the feathers' flexibility and act as anantimicrobial agent,inhibiting the growth of feather-degradingbacteria.^[345]This may be supplemented with the secretions offormic acidfrom ants, which birds receive through a behaviour known asanting,to remove feather parasites.^[346]
prenuptial moult: Seeprealternate moult.
primaries: Also,primary feathers;primary remiges.A type ofremexflight feather, they are connected to themanus(the bird's "hand", composed of carpometacarpus andphalanges); these are the longest and narrowest of the remiges (particularly those attached to the phalanges), and they can be individually rotated. These feathers are especially important for flapping flight, as they are the principal source ofthrust,moving the bird forward through the air. Most thrust is generated on the downstroke of flapping flight. However, on the upstroke (when the bird often draws its wing in close to its body), the primaries are separated and rotated, reducing air resistance while still helping to provide some thrust.^[347]The flexibility of the remiges on the wingtips of large soaring birds also allows for the spreading of those feathers, which helps to reduce the creation of wingtipvortices,thereby reducingdrag.^[348]In most flighted birds, some of the distalbarbuleson the innervaneof these feathers, calledfriction barbules,are specialized, with large lobular barbicels that are thought to help grip and prevent slippage of overlying feathers.^[349]Species vary in the number of primaries they possess. The number in non-passerines generally varies between 9 and 11,^[350]butgrebes,storksandflamingoshave 12 andostricheshave 16.^[351]While most modernpasserineshave ten primaries,^[350]some have only nine. Those with nine are missing the most distal primary, sometimes called theremicle,which is typically very small and sometimes rudimentary in passerines.^[351]
primary projection: Also,primary extension.The distance that a bird's longest primaries extend beyond its longest secondaries (ortertials) when its wings are folded.^[352]As withwing formulae,this measurement is useful for distinguishing between similarly plumaged birds; however, unlike wing formulae, it is not necessary to have the bird in-hand to make the measurement. Rather, this is a usefulrelativemeasurement—some species have long primary extensions, while others have shorter ones. Among theEmpidonaxflycatchersof the Americas, for example, thedusky flycatcherhas a much shorter primary extension than does the very similarly plumagedHammond's flycatcher.^[352]Europe'scommon skylarkhas a long primary projection, while that of the near-lookalikeOriental skylarkis very short.^[353]As a general rule, species which are long distance migrants will have longer primary projection than similar species which do not migrate or migrate shorter distances.^[354]
proventriculus: The first part of the stomach of a bird. The inside is lined with gastric glands, which secrete gastric juices containing enzymes and hydrochloric acid. In some birds, such as petrels, the proventriculus is expandable. This allows these birds to either digest their food later or carry it back to their young. The proventriculus leads into thegizzard.^[355]
psilopaedic: Young that are born naked or with only a small amount ofdownfeather coverage, as opposed to birds that areptilopaedic.^[16]
pterylae: Singular:pteryla.The feather tracts of a bird's skin from which thecontour feathersgrow, often in sharply defined and dense clusters—as opposed to theapterylae.^[23]See related:pterylosis.
pterylosis: Also,pterylography.The arrangement of feather tracts, as seen in a bird'spterylaeandapterylae,which varies across bird families and has been used in the past as a means for determining the evolutionary relationships among them.^[356]^[357]
ptilopaedic: Young that are born covered withdownfeathers, as opposed to birds that arepsilopaedic.^[16]
pupil: The dark disk at the centre of a bird's eye through which light enters, surrounded by the coloured outer ring of theiris.^[358]
pygostyle: Also,tail bone.A skeletal formation,plowshare-shaped in birds, in which the final fewcaudal vertebraeare fused into a singleossification,supporting the tailfeathersand musculature. In modernbirds,therectricesattach to these. The pygostyle is the main component of theuropygium.^[359]^[360]
pullus: (pluralpulli) A chick in the downy phase of development.^[361]

Q

quartering: A hunting technique where a bird flies slowly just above the water or ground in open habitats.^[362]
quill: Also,primary quill;main stem;scapus.The main stem of a feather from which all structures branch, if any. The proximal portion is called thecalamus or shaft,and the distal portion is called therachis,with the demarcation point between them usually defined as thesuperior umbilicus.^[61]^[363]However, some authorities define quill very differently, using it as a synonym for the calamus.^[364]The contradiction between sources on this issue is quite sharp. For example, in Asa Chandler's well known treatise,A study of the structure of feathers, with reference to their taxonomic significance,"quill" is defined at page 250 as: "The main stem of a feather, including both shaft and calamus (Coues, 1884; Beebe, 1906, et al.).Synonyms:main stem (Nitzsche, 1867) ".^[61]By contrast, in Frank B. Gill's well known treatise,Ornithology,he writes at page 80: "The hollow base of the shaft—the calamus, or quill—anchors the feather in a follicle below the surface of the skin".^[364]Inthis glossary,the former definition will be used, treating quill as denoting the "main stem" of a feather.

R

rachis: Plural:rachises;rachides.Also,shaft.Variable sp.:rhachis.^[365]Also defined:medulla.The distal or upper section of thequill,above thecalamus,stretching from thesuperior umbilicusto the tip of the feather, upon which thevanesand other structures are anchored. The rachis is flexible from side to side toward the blade projections, but far stiffer inward and outward from the bird's body. This arrangement aids in allowing theflight featherto act as a "resilient airfoil".^[363]The rachis is composed of two layers: a pithy, opaque material makes up the core layer called themedulla,which is covered by a thin and translucent outer layer orcortexthat features longitudinal internal ridges on the dorsal side and external ridges on the ventral side.^[8]
rectrices: Singular:rectrix.From the Latin for 'helmsman', they are the long, stiff, asymmetrically shaped, but symmetrically pairedpennaceous featherson the tail of a bird, which help it to brake and steer in flight. They lie in a single horizontal row on the rear margin of the anatomic tail. The vast majority of species having six pairs. They are absent in grebes and someratites,and greatly reduced in size in penguins.^[351]^[366]^[367]Manygrousespecies have more than 12 rectrices. Some species (includingruffed grouse,hazel grouseandcommon snipe) have a number that varies among individuals.^[368]Domestic pigeonshave a highly variable number, due to centuries of selective breeding.^[369]
remiges: Singular:remex.Also defined:postpatagium.From the Latin for 'oarsman', they are the long, stiff, asymmetrically shaped, but symmetrically pairedpennaceous featherson thewingsof a bird. They are located on theposteriorside of the wing.Ligamentsattach the longcalamifirmly to the wing bones, and a thick, strong band oftendinoustissue known as thepostpatagiumhelps to hold and support the remiges in place.^[370]Corresponding remiges on individual birds aresymmetricalbetween the two wings, matching to a large extent in size and shape (except in the case ofmutationor damage), though not necessarily in pattern.^[371]^[372]They are given different names depending on their position along the wing.
remicle: Also,little remex.The most distalprimary feather.The remicle is very small and usually non-functional—interpreted byChristian Ludwig Nitzschas "an aborted first primary". It was named by Richard S. Wray in 1887, who similarly thought of it as "representing an originally functional primary".^[373]
resident: Also,permanent resident;sedentary.Non-migratory;a bird that stays year-round and breeds in one geographic area or habitat.^[374]Bothpermanent residentandsedentaryare common with the defined usage. Althoughresidentis used in this sense, it is often used as well for birds that are "migratory in part of their breeding range and resident elsewhere",^[375]e.g., "Eurasian blackcaps(Sylvia atricapilla) are summer visitors to Eastern and Northern Europe, resident in Western Europe and winter visitors to Africa. "
rictal bristles: Also,bristles;bristle feathers;eyelashes.Stiff, hair-like, tapering feathers with a largerachisbut fewbarbsfound around the eyes and the base of thebeakof some bird species.^[376]They are common amonginsectivorousbirds, but are also found in some non-insectivorous species.^[377]They may serve a similar purpose toeyelashesandvibrissaeinmammals.Although there is as yet no clear evidence, it has been suggested that rictal bristles have sensory functions and may help insectivorous birds to capture prey.^[376]^[378]There is some experimental evidence to suggest that bristles may prevent particles from striking the eyes if, for example, a prey item is missed or breaks apart upon contact.^[377]
rosette: Also,gape rosette.A fleshy rosette found at the corners of the beaks of some birds, such as thepuffin.^[379]In the puffin, this is grown as part of its display plumage.^[380]
rhamphotheca: The outer surface of the beak consisting of a thin horny sheath ofkeratin,^[33]^[34]which can be subdivided into therhinothecaof theupper mandibleand thegnathothecaof thelower mandible.^[35]This covering arises from theMalpighian layerof the bird'sepidermis,^[35]growing from plates at the base of each mandible.^[381]There is avascularlayer between the rhamphotheca and the deeper layers of thedermis,which is attached directly to theperiosteumof the bones of the beak.^[382]The rhamphotheca grows continuously in most birds, and in some species, the colour varies seasonally.^[383]
rhinotheca: Therhamphotheca(thin horny sheath ofkeratin) covering theupper mandible.^[33]^[34]^[35]
rump: Also,uropygium;uropygial region;parson's nose,pope's nose;sultan's nose.Topographically,the region of a bird'supperpartsbetween the end of the back and the base of the tail.^[384]Anatomically,the fleshy protuberance visible at the posterior end of a bird. Its swollen appearance results from it housing theuropygial gland.^[385]See related:pygostyle.

S

scales: Specialized types:cancella;scutella;scutes.Also defined:reticulae;acrometatarsium / acrotarsium.Thescalesof birds are composed of keratin like their beaks and claws, and are found mainly on the toes andmetatarsus,though they may appear further up on the ankle. They do not overlap significantly, except in the cases ofkingfishersandwoodpeckers.The scales and scutes of birds were originally thought to behomologousto those of reptiles and mammals;^[386]later research, however, suggests that scales in birds re-evolved after the evolution of feathers.^[387]^[388]Bird embryos begin development with smooth skin. On the feet, thecorneum,or outermost layer of this skin may keratinize, thicken and form scales. Bird scales can be organized into:
i)cancella—minute scales which are really just a thickening and hardening of the skin, crisscrossed with shallow grooves;^[389]
ii)scutella—scales that are not quite as large as scutes, such as those found on thecaudal,or hind part, of the chickenmetatarsus;^[389]and
iii)scutes—the largest scales, usually on theanteriorsurface of the metatarsus anddorsalsurface of the toes. The rows of scutes on the anterior of the metatarsus is sometimes termed an "acrometatarsium" or "acrotarsium".^[389]
Reticulae,by contrast, are small, keeledscale-likestructures covering the bottoms of bird feet, but are not true scales. Evolutionary developmental studies on reticulae found that they are composed entirely ofAlpha -keratin(whereas, true epidermal scales are composed of a mix of Alpha and beta keratin).^[388]This, along with their unique structure, has led to the suggestion that reticulae are actually feather buds that were arrested early in development.^[92]^[388]^[390]See also:podotheca.
scapulars: Also,humeral region.Feathers covering a bird's scapula "at the base of the dorsal wing".^[391]
secondaries: Also,secondary feathers;secondary remiges.A type ofremexflight feather, they are connected to theulna.In some but not all bird species, the ligaments that bind secondaries to the bone connect to small, rounded projections that are called quill knobs. Secondary feathers remain close together in flight (they cannot be individually separated like the primaries can) and help to provide lift by creating the airfoil shape of the bird's wing. Secondaries tend to be shorter and broader than primaries, with blunter ends. They vary in number from six inhummingbirds,to as many as 40 in some species ofalbatross.In general, larger and longer-winged species have a larger number of secondaries.^[51]Birds in more than 40 non-passerinefamilies are missing the fifth secondary feather on each wing; a state known asdiastataxis.^[18]
semiplume: Also,semiplume feather.A type of feather that has features straddling the differences between truedownandcontourfeathers, such as by lacking cross-attachment between theirbarbs(i.e., without aflange–hookletanchoring system), but having a central, full-length shaft as in contour feathers. They are usually found on the margins of the pennaceousfeather tractsand, like down, provide insulative qualities and help fill out the plumage.Crest feathersare a type of semiplume.^[160]^[392]^[393]
sexual dimorphism: The common phenomenon amongst birds in which males and females of the same species exhibit different characteristics beyond the differences in their sexual organs. It can manifest in size or plumage differences. Sexual size dimorphism varies among taxa with males typically being larger, though this is not always the case, i.e., inbirds of prey,hummingbirdsand some species of flightless birds.^[394]^[395]Plumage dimorphism, in the form of ornamentation or colouration, also varies, though males are typically the more ornamented or brightly coloured sex.^[396]Such differences have been attributed to the unequal reproductive contributions of the sexes.^[397]
shaft: See entry forrachis.
sides: Thetopographicalarea between thebreastand theback,stretching from theaxilla(underarm) to theIliac crest.^[398]
song: A type of bird vocalization associated withcourtship and mating,tending to be longer and more complex than a bird'scall,which serve such functions as giving alarm or keeping members of aflockin contact.^[63]
speculum: Also,speculum feathers;mirror.A patch of usually brightly coloured feathers, often iridescent, on the innersecondary remigesin a number of duck species,^[399]though other birds may have them, such as the bright red or orange wing speculum of severalparrotsfrom the genusAmazona.^[400]
spur: Outgrowths ofbonecovered in a sheath ofhornwhich are found on some birds. Spurs are most commonly found on the feet or legs, though some birds possess spurs on the leading edge of the wings.^[401]
sternum: Plural:sterna.The breastbone of a bird. There are two types: i) carinate sterna—appearing in flighted birds, in which the ventral surface iskeel-shaped, which provides ample surface area for attachment of wing muscles used for flight; and ii) ratite sterna—appearing in flightless birds, such asrhea,in which the ventral surface is flattened.^[402]
submoustachial stripe: A usually light stripe of colour between themoustachial stripeabove, and thelateral throat-stripebelow.^[27]
supercilium: Plural:supercilia.Also,superciliary line;eyebrow / eye brow.A pale line appearing above the eye of a bird.^[97]Compareeye stripe.
superior umbilicus: Also,distal umbilicus.A very small opening on the ventral side (facing toward the bird's skin) of a feather's shaft, marking the distal end of thecalamusand the start of therachis.It is a remnant of the epithelial tube used by a bird's body to construct the feather.^[189]Compare:inferior umbilicus.
supraloral: Where a stripe is present only above thelores,and does not continue behind the eye, it is called asupraloral stripeor simplysupraloral,rather than thesupercilium.^[403]
syrinx: From the Greek word forpan pipes,σύριγξ, it is the vocal organ of birds. Located at the base of a bird'strachea,it produces sounds without thevocal foldsof mammals.^[404]As air flows through the syrinx, sound is produced by the vibration of some or all of themembrana tympaniformis(the walls of the syrinx) and a bar of cartilage connecting the dorsal and ventral extremities, known as thepessulus.This sets up a self-oscillating system that regulates the airflow used to create vocalizations. Sound modulation is achieved by varying the tension of muscles connected to the membranes and bronchial openings involved.^[405]The syrinx enables some species of birds (such asparrots,crowsandmynas) to mimic human speech. Unlike the mammalianlarynx,the syrinx is located where the trachea forks into the lungs. Thus,lateralization of bird songis possible, and somesongbirdscan produce more than one sound at a time.^[406]

T

tail: While the underlying structure of the tail is made up of bone (the pygostyle^[407]) and flesh, the term "tail" is most commonly used to refer to thefeathersgrowing from the region and the shape they form—that is,undertailanduppertailcoverts andrectrices,commonly radiating in a fan configuration.^[125]
tail coverts: See:undertail coverts,uppertail covertsandcrissum.
tail streamers: The elongated, narrow tips of thetailseen in some birds.^[408]They generally function as asexual ornament.^[409]In thebarn swallow,this has resulted in tail streamers about 12 mm (0.47 in) longer than is aerodynamically optimal.^[410]
talon: The claw of abird of prey;its primary hunting tool.^[411]The talons are very important; without them, most birds of prey would not be able to catch their food. Some birds also use claws for defensive purposes.Cassowariesuse claws on their inner toe (digit II) for defence, and have been known to disembowel people. All birds, however, have claws, which are used as general holdfasts and protection for the tip of the digits. Thehoatzinandturacoare unique amongextantbirds in having functional claws on the thumb and index finger (digit I and II) on the forelimbs as chicks, allowing them to climb trees until the adult plumage with flight feathers develop.^[412]^[413]However, several birds have a claw- or nail-like structure hidden under the feathers at the end of the hand digits, notably ostriches, emus, ducks, geese and kiwis.^[414]
tarsus: Also defined:tarsometatarsus.The third and most conspicuous portion of the bird's leg, from which the toes spring; the foot.^[415]In birds, the truetarsushas disappeared, with the proximal tarsals having fused with thetibia,the centralia having disappeared, and the distal bones having fused with themetatarsalsto form a singletarsometatarsusbone, effectively giving the leg a third segment.^[416]
teleoptiles: The collective term for all of the feathers of an adult bird.^[417]
temporal canthus: The area where the eyelids come together at the posterior corner of the eye (toward the sides of the head), as opposed to thenasal canthus.^[133]
tertiaries: Also,tertials;tertiary feathers;humeral feathers.A type of feather arising in the brachial region, i.e., "proximal to the innermost secondaries", usually growing in a grouping of three to four feathers.^[418]They are not considered trueremigesas they are not supported by attachment to the corresponding bone—in this case thehumerus.These elongated "true" tertials act as a protective cover for all or part of the foldedprimariesandsecondaries,and do not qualify as flight feathers as such.^[419]However, many authorities use the term tertials to refer to the shorter, more symmetrical innermost secondaries of passerines (arising from theolecranonand performing the same function as true tertials) in an effort to distinguish them from other secondaries. The termhumeralis sometimes used for birds such as the albatrosses and pelicans that have a long humerus.^[420]^[421]
throat: The triangular area of a bird's external anatomy, typically feathered, located between thechinand the upper part of thebreast.^[97]
thigh: Thetopographicalarea between the "knee" and the trunk of the body.^[422]
tibia: The usually feathered part of a bird's leg extending above the foot (tarsus).^[423]
tomia: Singular:tomium.The cutting edges of theupperandlowermandibles.^[36]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.^[92]Granivorous(seed-eating) birds, for example, have ridges in their tomia, which help the bird to slice through a seed's outerhull.^[424]Birds in roughly 30 families have tomia lined with tight bunches of very short bristles along their entire length. Most of these species are eitherinsectivores(preferring hard-shelled prey) orsnaileaters, and the brush-like projections may help to increase thecoefficient of frictionbetween the mandibles, thereby improving the bird's ability to hold hard prey items.^[425]Serrations onhummingbirdbills, 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 asnectarthieves, as they can more effectively hold and cut through long or waxyflower corollas.^[426]In some cases, the colour of a bird's tomia can help to distinguish between similar species. Thesnow goose,for example, has a reddish-pink bill with black tomia, while the whole beak of the similarRoss's gooseis pinkish-red, without darker tomia.^[427]
topography: The mapping of external features of bird anatomy.^[384]
torpor: Also,hypometabolism.An energy-conserving strategy used by some small birds [under 100 g (3.5 oz)] in which they become inactive and unreactive to external stimuli, and reduce theirmetabolismbelow their normal respiration and heart rate, causing decreases in body temperature by an average of 4–35 °C (39–95 °F). Two types of torpor are recognized. In one type, such as inhummingbirds,torpor may be entered on a daily basis and last only a few hours; other behaviours, such as foraging, continue during non-torpor periods. For other species, torpor may only be entered during cold conditions or when food becomes limited, but may persist for weeks or even months. The extent of unresponsiveness during torpor can be pronounced. A hummingbird in deep torpor, for example, with a body temperature of 18 °C (64 °F), will not respond to a variety of external stimuli, such as attempts to push it from a perch; only the locking reflex of the feet stops the bird from falling. Torpor has been reported in eightordersof birds.^[428]^[429]

U

undertail coverts: Covert featherscovering the base of thetailon its underside.^[125]Compare:uppertail coverts;contrastwing coverts.
underwing: The bottom side of a bird's wing; the hidden surface of the wing when it is folded.^[430]Compare:upperwing.
upper mandible: Also,maxilla.The upper part of a bird'sbillor beak, roughly corresponding to the upper jaw of mammals, it is supported by a three-prongedbonecalled the intermaxillary. The upper prong of this bone is embedded into the forehead, while the two lower prongs attach to the sides of theskull.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.^[33]The outer surface is covered in a thin, horny sheath ofkeratincalledrhamphotheca^[33]^[34]specially calledrhinothecain the upper mandible.^[35]Compare:lower mandible.
upper parts: Also,upper-partsandupperparts.Thetopographicalareas above the eyes of a bird (generally not including theeyestripe,if any) and through all of a bird's upper surface, including the "crown,mantle,back,tertialsandinner wing-coverts",^[431]although the term is sometimes used more inclusively, for all features from the bill to the tail.^[432]Compare:under parts.
under parts: Also,under-parts;underparts;lower parts;lower-parts;lowerparts.Alltopographicalareas on the underside or "under surface" of a bird,^[432]i.e., from thechinto the underside end of the tail (on the trunk and thus not including the legs).Compareupper parts.
uppertail coverts: Covert featherscovering the base of thetailon its upperside.^[125]Sometimes these coverts are more specialized. The "tail" of apeacockis actually very elongated uppertail coverts.^[117]Compareundertail coverts;contrastwing coverts.
upperwing: The top, or upper, side of a bird's wing.^[433]Compare:underwing.
uropygial gland: Also,preen gland;oil gland.A gland found at therump,at the base of thetailthat produces a waxy secretion made up of oils, fatty acids, fats and water that the majority of birds use in dailypreeningof their feathers. Though feathersmoultperiodically, they are inert structures without a nourishment system and would deteriorate rapidly without such application during preening. The applied waxy secretion aids in waterproofing, helps maintain feather moistness to keep them from becoming brittle and maintain flexibility, and is thought to promote the growth of nonpathogenic fungi, to deter feather lice and some forms of keratin-eating fungi and bacteria. The gland, a bilobed structure, is usually surrounded by a tuft of downy feathers that can act like a wick for the secreted oils.^[434]
uropygium: Seerump.

V

vagrant: Descriptive of a bird found outside its species' or subspecies' normal migration range or distribution area.^[435]
vane: Also,vexillum– plural:vexilla.The weblike expanse of flexible barbs, more or less interconnected by thehookletsof thebarbules,extending from each side of the distal part of the feather's shaft known as therachis.One side of the vane, called the outer vane, overlaps the inner vane side of the feather next to it.^[8]
vanule: The collective term for all of thebarbulesbranching from theramusof a feather'sbarb.^[28]
vaned feather: A classification term for pennaceous feathers (aka contour feathers) that clothe the outside of a bird's body.^[436]It refers to thevanedproperty of such feathers, i.e., having a flattened weblike expanse of flexiblebarbs,more or less interconnected by thehookletsof thebarbules,extending from each side of the distal part of the feather's shaft known as therachis.They make up one of three broad classes of feathers, the others beingdown feathers(which lie underneath vaned feathers, if present), and the hairlikefiloplumes.^[8]
vent: The external opening of thecloaca.^[437]
vent pecking: Anabnormal behaviourof birds in captivity, performed primarily by commercial egg-layinghens,characterized by pecking damage to thecloaca,the surrounding skin and underlying tissue.^[438]Vent pecking frequently occurs immediately after an egg has been laid when the cloaca often remains partly everted exposing themucosa.^[439]

W

wattle: Also,dewlap;lappet;palearia.Specialized type:rictal wattle.A type ofcaruncle,the wattle is a flap orlobeof flesh, often of a vivid colour, that hangs from the head of some birds, often from the chin or throat.^[440]^[441]When found at the corner of the mouth, they may be calledrictal wattles,such as inmasked lapwings.^[442]Birds with wattles includeturkeys,guineafowls,northern cassowaries,black-faced ibises,North Island kōkakos,wattled starlings(males, when innuptial plumage) and malebearded bellbirds,with the last having startling, beard-like wattles.^[443]^[444]
whiffling: A flight behaviour in which a bird rapidly descends with a zig-zagging,side-slippingmotion. During whiffling, some birds invert the aerodynamic mechanics of their bodies that normally providelift—flying by turning their bodies upside down but with their necks twisted 180 degrees to keep their heads in a normal flying position—resulting in a rapid plummet, before a quick arrest of the fall by resumption of a normal flying orientation.^[445]^[446]^[447]
wing bar: A stripe found on theupperwingthat is created by the contrast of the tips of the primary and secondary coverts.^[448]This feature may disappear when a birdmoultsand takes on a different seasonalplumage.^[449]Wing bars often occur in pairs.^[450]Contrast:speculum.
wing clipping: Also,wing trimming.The controversial practice of trimming a bird'sprimary wing feathersorremigesso that it is not fully flight-capable, until it grows replacement feathers during its nextmoult.^[451]If it is performed correctly, it is a painless procedure.^[452]Nevertheless, the practice can cause injury, e.g.,hemorrhagingas the result ofblood feathersbeing accidentally trimmed.^[453]
wing coverts: Also defined:upperwing coverts;underwing coverts;secondary coverts;primary coverts;greater(primary-/secondary-)coverts;median(primary-/secondary-)coverts;lesser(primary-/secondary-)coverts(the last being also calledbow coverts).
Covert feathersfound on a bird'swing.There are a number of classes and subclasses. Theupperwingcoverts fall into two groups: those on theinner wing,which overlay thesecondary flight feathers,are known as the secondary coverts, and those on theouter wing,which overlay theprimary flight feathers,are known as the primary coverts. Within each group, the feathers form a number of rows. The feathers of the outermost, largest, row are termed greater (primary-/secondary-) coverts; those in the next row are the median (primary-/secondary-) coverts, and any remaining rows are termed lesser (primary-/secondary-) coverts. Theunderwinghas corresponding sets of coverts (the names upperwing coverts and underwing coverts are used to distinguish the corresponding sets). In addition, the front edge of the wing is covered with a group of feathers called the marginal coverts. Within each group of wing coverts, the rows of feathers overlap each other likeroof tiles(the greater coverts are overlain by the median coverts, which in turn are overlain by the outermost row of lesser coverts, and so on).^[17]See also:axillaries;ear coverts;wing lining(marginal coverts).
wing formula: Amathematicalclassification of the shape of the distal end of a bird's wing. It can be used to help distinguish between species with similarplumages,and thus is particularly useful for those whoring (band)birds. To determine a bird's wing formula, the exposed distance between the tip of the most distalprimary,and the tip of itsgreater covert(the longest of the feathers that cover and protect the shaft of that primary), is measured in millimetres. In some cases, this results in a positive number (e.g., the primary extends beyond its greater covert), while in other cases it is a negative number (e.g., the primary is completely covered by the greater covert, as happens in some passerine species). Next, the longest primary feather is identified, and the differences in millimetres is measured between: i) the length of that primary; ii) that of all of the remaining primaries; and iii) of the longest secondary. If any primary shows a notch oremargination,this is noted, and the distance between the feather's tip and any notch is measured, as is the depth of the notch. All distance measurements are made with the bird's wing closed, so as to maintain the relative positions of the feathers. While there can be considerable variation across members of a species—and while the results are obviously impacted by the effects ofmoultand feather regeneration—even very closely related species show clear differences in their wing formulae.^[18]
wing lining: Also,marginal coverts.A grouping of very softcovert featherslining the anterior edge of the underside of the wing. Though the wing lining is composed of rows of these feathers, they are smooth in appearance such that it is often impossible to see the demarcation of the rows solely by eye.^[26]
wings: Wing types defined:elliptical;high speed;high aspect ratio;soaring.The bird'sforelimbsthat are the key to flight. Each wing has a central axis, composed of three limb bones: thehumerus,ulnaandradius.The hand, ormanus,which ancestrally was composed of five digits, is reduced to three digits (digit II, III and IV or I, II, III depending on the scheme followed^[454]), which serves as an anchor for theprimaries,one of two groups offlight feathersresponsible for the wing's airfoil shape. The other set of flight feathers, behind the carpal joint on the ulna, are called thesecondaries.The remaining feathers on the wing are known ascoverts,of which there are three sets. The wing sometimes has vestigial claws. In most species these are lost by the time the bird is adult (such as the highly visible ones used for active climbing byhoatzinchicks), but claws are retained into adulthood by thesecretarybird,screamers,finfoots,ostriches,severalswiftsand numerous others, as a local trait, in a few specimens. Bird wings are generally grouped into four types: i) elliptical wings—short, rounded and having a low aspect ratio, they allow for tight maneuvering in confined spaces such as might be found in dense vegetation. They are common in forest raptors, many migratory species ofpasserines,and in species such aspheasantsandpartridgesthat use a rapid take-off to evade predators. ii) High speed wings—short, pointed wings that, when combined with a heavy wing loading and rapid wingbeats, provide an energetically expensive high speed, as used by the bird with the fastest wing speed, theperegrine falcon.Mostducksandaukshave this type of wing but use the configuration for a different purpose, to "fly" underwater. iii) High aspect ratio wings—typified by low wing loading and being far longer than they are wide, they are used for slower flight, which may take the form of almost hovering (as used bykestrels,ternsandnightjars), or in soaring andglidingflight, particularly thedynamic soaringused byseabirds,which takes advantage of wind speed variation at different altitudes (wind shear) above ocean waves to provide lift. iv) Soaring wings with deep slots—common in larger species of inland birds, such aseagles,vultures,pelicansandstorks.The slots at the end of the wings, between the primaries, reduce theinduced dragandwingtip vortices,while the shorter size of the wings aids in takeoff (high aspect ratio wings require a longtaxito get airborne).^[455]
wingspan: The distance between each of the tips of the fully extendedwings.^[456]The living bird species with the largest wingspan is thewandering albatross,typically ranging from 2.5 to 3.5 m (8 ft 2 in to 11 ft 6 in).^[457]In contrast, the living bird species with the smallest wingspan is thebee hummingbird(also the smallest bird overall) with a wingspan of just 6.6 cm (2.6 in).^[458]
winter plumage: Seebasic plumage.

Z

zygodactylous: Also,yoke-toed.Also defined:heterodactylous.Descriptive of tetradactyl (four-toed) birds in which the architecture of the foot consists of two toes projecting forward and two toes projecting backward, such as inparrots,woodpeckersandcuckoos.In most birds with zygodactylous feet, the forward projecting toes are the second and third toes and the backward projecting toes are the fourth toe andhallux.However, introgonsalone, the third and fourth toes are in front, and the second and hallux are behind. This arrangement is sometimes separately designated, and termedheterodactylous.^[20]^[459]

Explanatory footnotes

^Ornithologists from theAmerican Museum of Natural Historysuggested in a 2016 article published inPLOS ONE,that the use of the "biological species concept"—classifying a grouping of birds as making up a singlespeciesby their ability to breed—is outdated and should be discarded in favour of a species' model that looks to similarity and dissimilarity in characteristics such as plumage, pattern and colouring to individuate bird species. Under such a model, it is suggested that there are ±18,000 individual living bird species in the world as of 2016.^[4]
^Emusandcassowariesare an exception in that theirafterfeathersare neither downy nor diminutive in relation to the vanes, but are comparable in size to them; such feathers are thus sometimes referred to as being "double plumed". The feathers ofostrichandrhea,also flightless birds, are by contrast "soft and filmy" in makeup, lacking anybarbulesto add the stiffness needed in workingflight feathers.^[7]
^Birds that undergo a second, non-breeding season moult (third moult overall) include:ptarmigans(Lagopus) (in the summer);long-tailed duck(Clangula hyemalis) (in the winter);ruff(Philomachus pugnax) (in the late winter or early spring); and may occur in other members of thesandpiper(Scolopacidae) family.^[31]
^In relation toflanksbeing defined as under parts, in the treatiseOrnithology in Laboratory and Field,the author states: "[a]lthough technically the sides of the body belong to both theupper partsand thelower parts,the imaginary line separating the two surfaces is so high on the trunk that the sides of the body are generally considered under parts only. "^[96]

Citations

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^Waters, Hannah (December 15, 2016)."New Study Doubles the World's Number of Bird Species By Redefining 'Species'".National Audubon Society. Archived fromthe originalon December 21, 2016.
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See also

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Citations

Bibliography