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Flight feathers (''Pennae volatus'') are the long, stiff, asymmetrically shaped, but symmetrically paired
pennaceous feather The pennaceous feather is a type of feather present in most modern birds and in some other species of maniraptoriform dinosaurs. Description A pennaceous feather has a stalk or quill. Its basal part, called a ''calamus'', is embedded in the skin ...
s on the
wing A wing is a type of fin that produces lift while moving through air or some other fluid. Accordingly, wings have streamlined cross-sections that are subject to aerodynamic forces and act as airfoils. A wing's aerodynamic efficiency is expres ...
s or tail of a bird; those on the wings are called remiges (), singular remex (), while those on the tail are called rectrices (), singular rectrix (). The primary function of the flight feathers is to aid in the generation of both
thrust Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that syst ...
and
lift Lift or LIFT may refer to: Physical devices * Elevator, or lift, a device used for raising and lowering people or goods ** Paternoster lift, a type of lift using a continuous chain of cars which do not stop ** Patient lift, or Hoyer lift, mobil ...
, thereby enabling
flight Flight or flying is the process by which an object moves through a space without contacting any planetary surface, either within an atmosphere (i.e. air flight or aviation) or through the vacuum of outer space (i.e. spaceflight). This can be a ...
. The flight feathers of some birds have evolved to perform additional functions, generally associated with territorial displays, courtship rituals or feeding methods. In some species, these feathers have developed into long showy plumes used in visual courtship displays, while in others they create a sound during display flights. Tiny serrations on the leading edge of their remiges help
owl Owls are birds from the order Strigiformes (), which includes over 200 species of mostly solitary and nocturnal birds of prey typified by an upright stance, a large, broad head, binocular vision, binaural hearing, sharp talons, and feathers a ...
s to fly silently (and therefore hunt more successfully), while the extra-stiff rectrices of
woodpecker Woodpeckers are part of the bird family Picidae, which also includes the piculets, wrynecks, and sapsuckers. Members of this family are found worldwide, except for Australia, New Guinea, New Zealand, Madagascar, and the extreme polar regions. ...
s help them to brace against tree trunks as they hammer on them. Even flightless birds still retain flight feathers, though sometimes in radically modified forms. The remiges are divided into primary and secondary feathers based on their position along the wing. There are typically 11 primaries attached to the manus (six attached to the metacarpus and five to the phalanges), but the outermost primary, called the remicle, is often rudimentary or absent; certain birds, notably the flamingos, grebes, and storks, have seven primaries attached to the metacarpus and 12 in all. Secondary feathers are attached to the ulna. The fifth secondary remex (numbered inwards from the carpal joint) was formerly thought to be absent in some species, but the modern view of this diastataxy is that there is a gap between the fourth and fifth secondaries. Tertiary feathers growing upon the adjoining portion of the brachium are not considered true remiges. The moult of their flight feathers can cause serious problems for birds, as it can impair their ability to fly. Different species have evolved different strategies for coping with this, ranging from dropping all their flight feathers at once (and thus becoming flightless for some relatively short period of time) to extending the moult over a period of several years.


Remiges

Remiges (from the Latin for "oarsman") are located on the posterior side of the wing.
Ligament A ligament is the fibrous connective tissue that connects bones to other bones. It is also known as ''articular ligament'', ''articular larua'', ''fibrous ligament'', or ''true ligament''. Other ligaments in the body include the: * Peritoneal li ...
s attach the long ''calami'' (quills) firmly to the wing bones, and a thick, strong band of tendinous tissue known as the ''postpatagium'' helps to hold and support the remiges in place. Corresponding remiges on individual birds are
symmetrical Symmetry (from grc, συμμετρία "agreement in dimensions, due proportion, arrangement") in everyday language refers to a sense of harmonious and beautiful proportion and balance. In mathematics, "symmetry" has a more precise definiti ...
between the two wings, matching to a large extent in size and shape (except in the case of
mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mi ...
or damage), though not necessarily in pattern. They are given different names depending on their position along the wing.


Primaries

Primaries are connected to the manus (the bird's "hand", composed of carpometacarpus and
phalanges The phalanges (singular: ''phalanx'' ) are digital bones in the hands and feet of most vertebrates. In primates, the thumbs and big toes have two phalanges while the other digits have three phalanges. The phalanges are classed as long bones. ...
); 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 of
thrust Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that syst ...
, moving the bird forward through the air. The mechanical properties of primaries are important in supporting flight. 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. 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 wingtip
vortices In fluid dynamics, a vortex ( : vortices or vortexes) is a region in a fluid in which the flow revolves around an axis line, which may be straight or curved. Vortices form in stirred fluids, and may be observed in smoke rings, whirlpools in th ...
, thereby reducing drag. The barbules on these feathers, friction barbules, are specialized with large lobular barbicels that help grip and prevent slippage of overlying feathers and are present in most of the flying birds. Species vary somewhat in the number of primaries they possess. The number in non-passerines generally varies between 9 and 11, but
grebe Grebes () are aquatic diving birds in the order Podicipediformes . Grebes are widely distributed freshwater birds, with some species also found in marine habitats during migration and winter. Some flightless species exist as well, most notably ...
s,
stork Storks are large, long-legged, long-necked wading birds with long, stout bills. They belong to the family called Ciconiidae, and make up the order Ciconiiformes . Ciconiiformes previously included a number of other families, such as herons an ...
s and
flamingo Flamingos or flamingoes are a type of Wader, wading bird in the Family (biology), family Phoenicopteridae, which is the only extant family in the order Phoenicopteriformes. There are four flamingo species distributed throughout the Americas ...
s have 12, and
ostrich Ostriches are large flightless birds of the genus ''Struthio'' in the order Struthioniformes, part of the infra-class Palaeognathae, a diverse group of flightless birds also known as ratites that includes the emus, rheas, and kiwis. There are ...
es have 16. While most modern
passerine A passerine () is any bird of the order Passeriformes (; from Latin 'sparrow' and '-shaped'), which includes more than half of all bird species. Sometimes known as perching birds, passerines are distinguished from other orders of birds by t ...
s have ten primaries, some have only nine. Those with nine are missing the most distal primary (sometimes called the remicle) which is typically very small and sometimes rudimentary in passerines. The outermost primaries—those connected to the phalanges—are sometimes known as pinions.


Secondaries

Secondaries are connected to the
ulna The ulna (''pl''. ulnae or ulnas) is a long bone found in the forearm that stretches from the elbow to the smallest finger, and when in anatomical position, is found on the medial side of the forearm. That is, the ulna is on the same side of t ...
. In some species, the ligaments that bind these remiges to the bone connect to small, rounded projections, known as
quill knob The pennaceous feather is a type of feather present in most modern birds and in some other species of maniraptoriform dinosaurs. Description A pennaceous feather has a stalk or quill. Its basal part, called a ''calamus'', is embedded in the skin ...
s, on the ulna; in other species, no such knobs exist. 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 (see illustration). They vary in number from 6 in
hummingbird Hummingbirds are birds native to the Americas and comprise the biological family Trochilidae. With about 361 species and 113 genera, they occur from Alaska to Tierra del Fuego, but the vast majority of the species are found in the tropics aro ...
s to as many as 40 in some species of
albatross Albatrosses, of the biological family Diomedeidae, are large seabirds related to the procellariids, storm petrels, and diving petrels in the order Procellariiformes (the tubenoses). They range widely in the Southern Ocean and the North Pacifi ...
. In general, larger and longer-winged species have a larger number of secondaries. Birds in more than 40 non-passerine families seem to be missing the fifth secondary feather on each wing, a state known as diastataxis (those that do have the fifth secondary are said to eutaxic). In these birds, the fifth set of secondary
covert feather A covert feather or tectrix on a bird is one of a set of feathers, called coverts (or ''tectrices''), which, as the name implies, cover other feathers. The coverts help to smooth airflow over the wings and tail. Ear coverts The ear coverts are sm ...
s does not cover any remiges, possibly due to a twisting of the feather papillae during
embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male spe ...
nic development. Loons, grebes,
pelican Pelicans (genus ''Pelecanus'') are a genus of large water birds that make up the family Pelecanidae. They are characterized by a long beak and a large throat pouch used for catching prey and draining water from the scooped-up contents before s ...
s,
hawk Hawks are bird of prey, birds of prey of the family Accipitridae. They are widely distributed and are found on all continents except Antarctica. * The subfamily Accipitrinae includes goshawks, sparrowhawks, sharp-shinned hawks and others. Th ...
s and
eagle Eagle is the common name for many large birds of prey of the family Accipitridae. Eagles belong to several groups of genera, some of which are closely related. Most of the 68 species of eagle are from Eurasia and Africa. Outside this area, just ...
s, cranes,
sandpiper Sandpipers are a large family, Scolopacidae, of waders. They include many species called sandpipers, as well as those called by names such as curlew and snipe. The majority of these species eat small invertebrates picked out of the mud or soil ...
s,
gull Gulls, or colloquially seagulls, are seabirds of the family Laridae in the suborder Lari. They are most closely related to the terns and skimmers and only distantly related to auks, and even more distantly to waders. Until the 21st century, m ...
s, parrots, and owls are among the families missing this feather.


Tertials

Tertials arise in the brachial region and are not considered true remiges as they are not supported by attachment to the corresponding bone, in this case the humerus. These elongated "true" tertials act as a protective cover for all or part of the folded primaries and secondaries, and do not qualify as flight feathers as such. However, many authorities use the term tertials to refer to the shorter, more symmetrical innermost secondaries of passerines (arising from the
olecranon The olecranon (, ), is a large, thick, curved bony eminence of the ulna, a long bone in the forearm that projects behind the elbow. It forms the most pointed portion of the elbow and is opposite to the cubital fossa or elbow pit. The olecranon ...
and performing the same function as true tertials) in an effort to distinguish them from other secondaries. The term
humeral The humerus (; ) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius and ulna, and consists of three sections. The humeral upper extremity consists of a rou ...
is sometimes used for birds such as the albatrosses and pelicans that have a long humerus.


Tectrices

The calami of the flight feathers are protected by a layer of non-flight feathers called ''covert'' feathers or '' tectrices'' (singular ''tectrix''), at least one layer of them both above and beneath the flight feathers of the wings as well as above and below the rectrices of the tail. These feathers may vary widely in size – in fact, the upper tail tectrices of the male
peafowl Peafowl is a common name for three bird species in the genera '' Pavo'' and ''Afropavo'' within the tribe Pavonini of the family Phasianidae, the pheasants and their allies. Male peafowl are referred to as peacocks, and female peafowl are refe ...
, rather than its rectrices, are what constitute its elaborate and colorful "train".


Emargination

The outermost primaries of large soaring birds, particularly raptors, often show a pronounced narrowing at some variable distance along the feather edges. These narrowings are called either notches or emarginations depending on the degree of their slope. 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 remex. (Both are visible on the primary in the photo showing the feathers; they can be found about halfway along both sides of the left hand feather—a shallow notch on the left, and a gradual emargination on the right.) The presence of notches and emarginations creates gaps at the wingtip; air is forced through these gaps, increasing the generation of lift.


Alula

Feathers on the
alula The alula , or bastard wing, (plural ''alulae'') is a small projection on the anterior edge of the wing of modern birds and a few non-avian dinosaurs. The word is Latin and means "winglet"; it is the diminutive of ''ala'', meaning "wing". The al ...
or bastard wing are not generally considered to be flight feathers in the strict sense; though they are asymmetrical, they lack the length and stiffness of most true flight feathers. However, alula feathers are definitely an aid to slow flight. These feathers—which are attached to the bird's "thumb" and normally lie flush against the
anterior Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...
edge of the wing—function in the same way as the
slats Super Low Altitude Test Satellite (SLATS) or Tsubame was a JAXA satellite intended to demonstrate operations in very low Earth orbit (VLEO, below 200 km), using ion engines to counteract aerodynamic drag from the Earth's atmosphere which i ...
on an airplane wing, allowing the wing to achieve a higher than normal
angle of attack In fluid dynamics, angle of attack (AOA, α, or \alpha) is the angle between a reference line on a body (often the chord line of an airfoil) and the vector representing the relative motion between the body and the fluid through which it is m ...
 – and thus
lift Lift or LIFT may refer to: Physical devices * Elevator, or lift, a device used for raising and lowering people or goods ** Paternoster lift, a type of lift using a continuous chain of cars which do not stop ** Patient lift, or Hoyer lift, mobil ...
 – without resulting in a stall. By manipulating its thumb to create a gap between the alula and the rest of the wing, a bird can avoid stalling when flying at low speeds or landing.


Delayed development in hoatzins

The development of the remiges (and alulae) of nestling
hoatzin The hoatzin ( ) or hoactzin ( ), (''Opisthocomus hoazin''), is the only species in the order Opisthocomiformes. It is a species of tropical bird found in swamps, riparian forests, and mangroves of the Amazon and the Orinoco basins in South Ameri ...
s is much delayed compared to the development of these feathers in other young birds, presumably because young hoatzins are equipped with
claw A claw is a curved, pointed appendage found at the end of a toe or finger in most amniotes (mammals, reptiles, birds). Some invertebrates such as beetles and spiders have somewhat similar fine, hooked structures at the end of the leg or tarsus ...
s on their first two digits. They use these small rounded hooks to grasp branches when clambering about in trees, and feathering on these digits would presumably interfere with that functionality. Most youngsters shed their claws sometime between their 70th and 100th day of life, but some retain them— though
callus A callus is an area of thickened and sometimes hardened skin that forms as a response to repeated friction, pressure, or other irritation. Since repeated contact is required, calluses are most often found on the feet and hands, but they may o ...
ed-over and unusable— into adulthood.


Rectrices

Rectrices (singular rectrix) from the Latin word for "helmsman", help the bird to brake and steer in flight. These feathers lie in a single horizontal row on the rear margin of the anatomic tail. Only the central pair are attached (via
ligament A ligament is the fibrous connective tissue that connects bones to other bones. It is also known as ''articular ligament'', ''articular larua'', ''fibrous ligament'', or ''true ligament''. Other ligaments in the body include the: * Peritoneal li ...
s) to the tail bones; the remaining rectrices are embedded into the rectricial bulbs, complex structures of fat and muscle that surround those bones. Rectrices are always paired, with a vast majority of species having six pairs. They are absent in grebes and some
ratite A ratite () is any of a diverse group of flightless, large, long-necked, and long-legged birds of the infraclass Palaeognathae. Kiwi, the exception, are much smaller and shorter-legged and are the only nocturnal extant ratites. The systematics ...
s, and greatly reduced in size in penguins. Many
grouse Grouse are a group of birds from the order Galliformes, in the family Phasianidae. Grouse are presently assigned to the tribe Tetraonini (formerly the subfamily Tetraoninae and the family Tetraonidae), a classification supported by mitochondr ...
species have more than 12 rectrices. In some species (including
ruffed grouse The ruffed grouse (''Bonasa umbellus'') is a medium-sized grouse occurring in forests from the Appalachian Mountains across Canada to Alaska. It is the most widely distributed game bird in North America. It is non-migratory. It is the only specie ...
,
hazel grouse The hazel grouse (''Tetrastes bonasia''), sometimes called the hazel hen, is one of the smaller members of the grouse family of birds. It is a sedentary species, breeding across the Palearctic as far east as Hokkaido, and as far west as eastern a ...
and
common snipe The common snipe (''Gallinago gallinago'') is a small, stocky wader native to the Old World. The breeding habitats are marshes, bogs, tundra and wet meadows throughout the Palearctic. In the north, the distribution limit extends from Iceland ...
), the number varies among individuals.
Domestic pigeon The domestic pigeon (''Columba livia domestica'' or ''Columba livia'' ''forma'' ''domestica'') is a pigeon subspecies that was derived from the rock dove (also called the rock pigeon). The rock pigeon is the world's oldest domesticated bird. Me ...
s have a highly variable number as a result of changes brought about over centuries of selective breeding.


Numbering conventions

In order to make the discussion of such topics as
moult In biology, moulting (British English), or molting (American English), also known as sloughing, shedding, or in many invertebrates, ecdysis, is the manner in which an animal routinely casts off a part of its body (often, but not always, an outer ...
processes or
body Body may refer to: In science * Physical body, an object in physics that represents a large amount, has mass or takes up space * Body (biology), the physical material of an organism * Body plan, the physical features shared by a group of anima ...
structure easier,
ornithologist Ornithology is a branch of zoology that concerns the "methodological study and consequent knowledge of birds with all that relates to them." Several aspects of ornithology differ from related disciplines, due partly to the high visibility and th ...
s assign a number to each flight feather. By convention, the numbers assigned to primary feathers always start with the letter P ''(P1, P2, P3, etc.)'', those of secondaries with the letter S, those of tertials with T and those of rectrices with R. Most authorities number the primaries descendantly, starting from the innermost primary (the one closest to the secondaries) and working outwards; others number them ascendantly, from the most distal primary inwards. There are some advantages to each method. Descendant numbering follows the normal sequence of most birds' primary moult. In the event that a species is missing the small distal 10th primary, as some passerines are, its lack does not impact the numbering of the remaining primaries. Ascendant numbering, on the other hand, allows for uniformity in the numbering of non-passerine primaries, as they almost invariably have four attached to the manus regardless of how many primaries they have overall. This method is particularly useful for indicating wing formulae, as the outermost primary is the one with which the measurements begin. Secondaries are always numbered ascendantly, starting with the outermost secondary (the one closest to the primaries) and working inwards. Tertials are also numbered ascendantly, but in this case, the numbers continue on consecutively from that given to the last secondary (e.g. ... S5, S6, T7, T8, ... etc.). Rectrices are always numbered from the centermost pair outwards in both directions.


Specialized flight feathers

The flight feathers of some species have undergone evolutionary changes which allow them to provide additional functionality. In some species, for example, either remiges or rectrices make a sound during flight. These sounds are most often associated with courtship or territorial displays. The outer primaries of male
broad-tailed hummingbird The broad-tailed hummingbird (''Selasphorus platycercus'') is a medium-sized hummingbird species found in highland regions from western United States and Western Canada to Mexico and Guatemala. Description Medium in size, the broad-tailed ...
s produce a distinctive high-pitched trill, both in direct flight and in power-dives during courtship displays; this trill is diminished when the outer primaries are worn, and absent when those feathers have been moulted. During the
northern lapwing The northern lapwing (''Vanellus vanellus''), also known as the peewit or pewit, tuit or tew-it, green plover, or (in Ireland and Britain) pyewipe or just lapwing, is a bird in the lapwing subfamily. It is common through temperate Eurosiberia. ...
's zigzagging display flight, the bird's outer primaries produce a humming sound. The outer primaries of the male
American woodcock The American woodcock (''Scolopax minor''), sometimes colloquially referred to as the timberdoodle, the bogsucker, the hokumpoke, and the Labrador twister, is a small shorebird species found primarily in the eastern half of North America. Woodcoc ...
are shorter and slightly narrower than those of the female, and are likely the source of the whistling and twittering sounds made during his courtship display flights. Male
club-winged manakin The club-winged manakin (''Machaeropterus deliciosus'') is a small passerine bird which is a resident breeding species in the cloud forest on the western slopes of the Andes Mountains of Colombia and northwestern Ecuador. The manakins are a famil ...
s use modified secondaries to make a clear trilling courtship call. A curve-tipped secondary on each wing is dragged against an adjacent ridged secondary at high speeds (as many as 110 times per second—slightly faster than a hummingbird's wingbeat) to create a
stridulation Stridulation is the act of producing sound by rubbing together certain body parts. This behavior is mostly associated with insects, but other animals are known to do this as well, such as a number of species of fish, snakes and spiders. The mech ...
much like that produced by some insects. Both Wilson's and
common snipe The common snipe (''Gallinago gallinago'') is a small, stocky wader native to the Old World. The breeding habitats are marshes, bogs, tundra and wet meadows throughout the Palearctic. In the north, the distribution limit extends from Iceland ...
have modified outer tail feathers which make noise when they are spread during the birds' roller coaster display flights; as the bird dives, wind flows through the modified feathers and creates a series of rising and falling notes, which is known as "winnowing". Differences between the sounds produced by these two former conspecific subspecies—and the fact that the outer two pairs of rectrices in Wilson's snipe are modified, while only the single outermost pair are modified in common snipe—were among the characteristics used to justify their splitting into two distinct and separate species. Flight feathers are also used by some species in visual displays. Male standard-winged and
pennant-winged nightjar The pennant-winged nightjar (''Caprimulgus vexillarius'') is a species of nightjar that occurs from Nigeria to northern South Africa. It is an intra-African migrant and displays remarkable sexual dimorphism in the breeding season. Range and ha ...
s have modified P2 primaries (using the descendant numbering scheme explained above) which are displayed during their courtship rituals. In the standard-winged nightjar, this modified primary consists of an extremely long shaft with a small "pennant" (actually a large web of barbules) at the tip. In the pennant-winged nightjar, the P2 primary is an extremely long (but otherwise normal) feather, while P3, P4 and P5 are successively shorter; the overall effect is a broadly forked wingtip with a very long plume beyond the lower half of the fork. Males of many species, ranging from the widely introduced
ring-necked pheasant The common pheasant (''Phasianus colchicus'') is a bird in the pheasant family (Phasianidae). The genus name comes from Latin ''phasianus'', "pheasant". The species name ''colchicus'' is Latin for "of Colchis" (modern day Georgia), a country on ...
to Africa's many whydahs, have one or more elongated pairs of rectrices, which play an often-critical role in their courtship rituals. The outermost pair of rectrices in male
lyrebird A lyrebird is either of two species of ground-dwelling Australian birds that compose the genus ''Menura'', and the family Menuridae. They are most notable for their impressive ability to mimic natural and artificial sounds from their environ ...
s are extremely long and strongly curved at the ends. These plumes are raised up over the bird's head (along with a fine spray of modified uppertail coverts) during his extraordinary display. Rectrix modification reaches its pinnacle among the
birds of paradise The birds-of-paradise are members of the family Paradisaeidae of the order Passeriformes. The majority of species are found in eastern Indonesia, Papua New Guinea and eastern Australia. The family has 44 species in 17 genera. The members of ...
, which display an assortment of often bizarrely modified feathers, ranging from the extremely long plumes of the
ribbon-tailed astrapia The ribbon-tailed astrapia, also known as Shaw Mayer's astrapia (''Astrapia mayeri''), is a species of bird-of-paradise. The ribbon-tailed astrapia is distributed and endemic to subalpine forests in western part of the central highlands of Papua ...
(nearly three times the length of the bird itself) to the dramatically coiled twin plumes of the
magnificent bird-of-paradise The magnificent bird-of-paradise (''Cicinnurus magnificus'') is a species of bird-of-paradise. The magnificent bird-of-paradise is evaluated as Least Concern on the IUCN Red List of Threatened Species. They are listed on Appendix II of CITES. Et ...
.
Owl Owls are birds from the order Strigiformes (), which includes over 200 species of mostly solitary and nocturnal birds of prey typified by an upright stance, a large, broad head, binocular vision, binaural hearing, sharp talons, and feathers a ...
s have remiges which are serrated rather than smooth on the leading edge. This adaptation disrupts the flow of air over the wings, eliminating the noise that airflow over a smooth surface normally creates, and allowing the birds to fly and hunt silently. The rectrices of
woodpecker Woodpeckers are part of the bird family Picidae, which also includes the piculets, wrynecks, and sapsuckers. Members of this family are found worldwide, except for Australia, New Guinea, New Zealand, Madagascar, and the extreme polar regions. ...
s are proportionately short and very stiff, allowing them to better brace themselves against tree trunks while feeding. This adaptation is also found, though to a lesser extent, in some other species that feed along tree trunks, including
woodcreeper The woodcreepers (Dendrocolaptinae) comprise a subfamily of suboscine passerine birds endemic to the Neotropics. They have traditionally been considered a distinct family Dendrocolaptidae, but most authorities now place them as a subfamily of th ...
s and
treecreeper The treecreepers are a family, Certhiidae, of small passerine birds, widespread in wooded regions of the Northern Hemisphere and sub-Saharan Africa. The family contains eleven species in two genera, '' Certhia'' and '' Salpornis''. Their plumage ...
s. Scientists have not yet determined the function of all flight feather modifications. Male swallows in the genera '' Psalidoprocne'' and '' Stelgidopteryx'' have tiny recurved hooks on the leading edges of their outer primaries, but the function of these hooks is not yet known; some authorities suggest they may produce a sound during territorial or courtship displays.


Vestigiality in flightless birds

Over time, a small number of bird species have lost their ability to fly. Some of these, such as the steamer ducks, show no appreciable changes in their flight feathers. Some, such as the
Titicaca grebe The Titicaca grebe (''Rollandia microptera''), also known as the Titicaca flightless grebe or short-winged grebe, is a grebe found on the altiplano of Peru and Bolivia. As its name implies, its main population occurs on Lake Titicaca. Lake Uru ...
and a number of the flightless rails, have a reduced number of primaries. The remiges of ratites are soft and downy; they lack the interlocking hooks and barbules that help to stiffen the flight feathers of other birds. In addition, the
emu The emu () (''Dromaius novaehollandiae'') is the second-tallest living bird after its ratite relative the ostrich. It is endemic to Australia where it is the largest native bird and the only extant member of the genus ''Dromaius''. The emu' ...
's remiges are proportionately much reduced in size, while those of the
cassowaries Cassowaries ( tpi, muruk, id, kasuari) are flightless birds of the genus ''Casuarius'' in the order Casuariiformes. They are classified as ratites (flightless birds without a keel (bird anatomy), keel on their sternum bones) and are native t ...
are reduced both in number and structure, consisting merely of 5–6 bare quills. Most ratites have completely lost their rectrices; only the ostrich still has them. Penguins have lost their differentiated flight feathers. As adults, their wings and tail are covered with the same small, stiff, slightly curved feathers as are found on the rest of their bodies. The ground-dwelling kakapo, which is the world's only flightless parrot, has remiges which are shorter, rounder and more symmetrically vaned than those of parrots capable of flight; these flight feathers also contain fewer interlocking barbules near their tips.


Moult

Once they have finished growing, feathers are essentially dead structures. Over time, they become worn and abraded, and need to be replaced. This replacement process is known as
moult In biology, moulting (British English), or molting (American English), also known as sloughing, shedding, or in many invertebrates, ecdysis, is the manner in which an animal routinely casts off a part of its body (often, but not always, an outer ...
(molt in the United States). The loss of wing and tail feathers can affect a bird's ability to fly (sometimes dramatically) and in certain
families Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Ideal ...
can impair the ability to feed or perform
courtship Courtship is the period wherein some couples get to know each other prior to a possible marriage. Courtship traditionally may begin after a betrothal and may conclude with the celebration of marriage. A courtship may be an informal and private m ...
displays. The timing and progression of flight feather moult therefore varies among families. For most birds, moult begins at a certain specific point, called a focus (plural foci), on the wing or tail and proceeds in a sequential manner in one or both directions from there. For example, most passerines have a focus between the innermost primary (P1, using the numbering scheme explained above) and outermost secondary (S1), and a focus point in the middle of the center pair of rectrices. As passerine moult begins, the two feathers closest to the focus are the first to drop. When replacement feathers reach roughly half of their eventual length, the next feathers in line (P2 and S2 on the wing, and both R2s on the tail) are dropped. This pattern of drop and replacement continues until moult reaches either end of the wing or tail. The speed of the moult can vary somewhat within a species. Some passerines that breed in the
Arctic The Arctic ( or ) is a polar regions of Earth, polar region located at the northernmost part of Earth. The Arctic consists of the Arctic Ocean, adjacent seas, and parts of Canada (Yukon, Northwest Territories, Nunavut), Danish Realm (Greenla ...
, for example, drop many more flight feathers at once (sometimes becoming briefly flightless) in order to complete their entire wing moult prior to migrating south, while those same species breeding at lower
latitude In geography, latitude is a coordinate that specifies the north– south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north pol ...
s undergo a more protracted moult. In many species, there is more than one focus along the wing. Here, moult begins at all foci simultaneously, but generally proceeds only in one direction. Most grouse, for example, have two wing foci: one at the wingtip, the other between feathers P1 and S1. In this case, moult proceeds descendantly from both foci. Many large, long-winged birds have multiple wing foci. Birds that are heavily "wing-loaded"—that is, heavy-bodied birds with relatively short wings—have great difficulty flying with the loss of even a few flight feathers. A protracted moult like the one described above would leave them vulnerable to
predator Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill th ...
s for a sizeable portion of the year. Instead, these birds lose all their flight feathers at once. This leaves them completely flightless for a period of three to four weeks, but means their overall period of vulnerability is significantly shorter than it would otherwise be. Eleven families of birds, including loons, grebes and most
waterfowl Anseriformes is an order of birds also known as waterfowl that comprises about 180 living species of birds in three families: Anhimidae (three species of screamers), Anseranatidae (the magpie goose), and Anatidae, the largest family, which in ...
, have this moult strategy. The cuckoos show what is called saltatory or transilient wing moults. In simple forms, this involves the moulting and replacement of odd-numbered primaries and then the even-numbered primaries. There are however complex variations with differences based on life history. Arboreal
woodpecker Woodpeckers are part of the bird family Picidae, which also includes the piculets, wrynecks, and sapsuckers. Members of this family are found worldwide, except for Australia, New Guinea, New Zealand, Madagascar, and the extreme polar regions. ...
s, which depend on their tails—particularly the strong central pair of rectrices—for support while they feed, have a unique tail moult. Rather than moulting their central tail feathers first, as most birds do, they retain these feathers until last. Instead, the second pair of rectrices (both R2 feathers) are the first to drop. (In some species in the genera ''Celeus'' and ''Dendropicos'', the third pair is the first dropped.) The pattern of feather drop and replacement proceeds as described for passerines (above) until all other rectrices have been replaced; only then are the central tail rectrices moulted. This provides some protection to the growing feathers, since they're always covered by at least one existing feather, and also ensures that the bird's newly strengthened tail is best able to cope with the loss of the crucial central rectrices. Ground-feeding woodpeckers, such as the
wryneck The wrynecks (genus ''Jynx'') are a small but distinctive group of small Old World woodpeckers. ''Jynx'' is from the Ancient Greek ''iunx'', the Eurasian wryneck. These birds get their English name from their ability to turn their heads almos ...
s, do not have this modified moult strategy; in fact, wrynecks moult their outer tail feathers first, with moult proceeding
proximal Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...
ly from there.


Age differences in flight feathers

There are often substantial differences between the remiges and rectrices of adults and juveniles of the same species. Because all juvenile feathers are grown at once—a tremendous energy burden to the developing bird—they are softer and of poorer quality than the equivalent feathers of adults, which are moulted over a longer period of time (as long as several years in some cases). As a result, they wear more quickly. As feathers grow at variable rates, these variations lead to visible dark and light bands in the fully formed feather. These ''growth bars'' and their widths have been used to determine the daily nutritional status of birds. Each light and dark bar correspond to around 24 hours and the use of this technique has been called ''ptilochronology'' (analogous to
dendrochronology Dendrochronology (or tree-ring dating) is the scientific method of dating tree rings (also called growth rings) to the exact year they were formed. As well as dating them, this can give data for dendroclimatology, the study of climate and atmos ...
). In general, juveniles have feathers which are narrower and more sharply pointed at the tip. This can be particularly visible when the bird is in flight, especially in the case of raptors. The trailing edge of the wing of a juvenile bird can appear almost serrated, due to the feathers' sharp tips, while that of an older bird will be straighter-edged. The flight feathers of a juvenile bird will also be uniform in length, since they all grew at the same time. Those of adults will be of various lengths and levels of wear, since each is moulted at a different time. The flight feathers of adults and juveniles can differ considerably in length, particularly among the raptors. Juveniles tend to have slightly longer rectrices and shorter, broader wings (with shorter outer primaries, and longer inner primaries and secondaries) than do adults of the same species. However, there are many exceptions. In longer-tailed species, such as
swallow-tailed kite The swallow-tailed kite (''Elanoides forficatus'') is a pernine raptor which breeds from the southeastern United States to eastern Peru and northern Argentina. It is the only species in the genus ''Elanoides''. Most North and Central American b ...
,
secretary bird The secretarybird or secretary bird (''Sagittarius serpentarius'') is a large, mostly terrestrial bird of prey. Endemic to Africa, it is usually found in the open grasslands and savanna of the sub-Saharan region. John Frederick Miller describe ...
and
European honey buzzard The European honey buzzard (''Pernis apivorus''), also known as the pern or common pern, is a bird of prey in the family Accipitridae. Etymology Despite its English name, this species is more closely related to kites of the genera '' Leptodon'' a ...
, for example, juveniles have shorter rectrices than adults do. Juveniles of some ''Buteo'' buzzards have narrower wings than adults do, while those of large juvenile falcons are longer. It is theorized that the differences help young birds compensate for their inexperience, weaker flight muscles and poorer flying ability.


Wing formula

A wing formula describes the shape of distal end of a bird's wing in a
mathematical Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in modern mathematics ...
way. It can be used to help distinguish between species with similar plumages, and thus is particularly useful for those who
ring (band) Ring were an English psychedelic rock band active during the 1980s. The band is notable for having helped to launch the subsequent musical careers of Robert White (Levitation, The Milk And Honey Band), Michael Tubb (also of The Milk And Honey ...
birds. To determine a bird's wing formula, the distance between the tip of the most distal primary and the tip of its greater covert (the longest of the feathers that cover and protect the shaft of that primary) is measured in millimeters. 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 between the length of that primary and that of all remaining primaries and of the longest secondary are also measured, again in millimeters. If any primary shows a notch or emargination, 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 of moult and feather regeneration—even very closely related species show clear differences in their wing formulas.


Primary extension

The distance that a bird's longest primaries extend beyond its longest secondaries (or tertials) when its wings are folded is referred to as the primary extension or primary projection. As with wing 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 useful ''relative'' measurement—some species have long primary extensions, while others have shorter ones. Among the ''
Empidonax The genus ''Empidonax'' is a group of small insect-eating passerine birds in the tyrant flycatcher family, the Tyrannidae. The genus name ''Empidonax'' is from Ancient Greek ''empis'', "gnat", and ''anax'', "master". Most of these birds are ver ...
'' flycatchers of the Americas, for example, the dusky flycatcher has a much shorter primary extension than does the very similarly plumaged
Hammond's flycatcher Hammond's flycatcher (''Empidonax hammondii'') is a flycatcher in the family Tyrannidae. This small insectivorous bird inhabits the coniferous and mixed forests of western North America. The name of this bird commemorates William Alexander Hammon ...
. Europe's
common skylark The Eurasian skylark (''Alauda arvensis'') is a passerine bird in the lark family, Alaudidae. It is a widespread species found across Europe and the Palearctic with introduced populations in New Zealand, Australia and on the Hawaiian Islands. ...
has a long primary projection, while that of the near-lookalike
Oriental skylark The Oriental skylark (''Alauda gulgula''), also known as the small skylark, is a species of skylark found in the southern, central and eastern Palearctic . Like other skylarks, it is found in open grassland where it feeds on seeds and insects. ...
is very short. 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.


See also

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Bird anatomy Bird anatomy, or the physiological structure of birds' bodies, shows many unique adaptations, mostly aiding flight. Birds have a light skeletal system and light but powerful musculature which, along with circulatory and respiratory systems capabl ...
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Bird flight Bird flight is the primary mode of locomotion used by most bird species in which birds take off and fly. Flight assists birds with feeding, breeding, avoiding predators, and migrating. Bird flight is one of the most complex forms of locomotion ...
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Drumming (snipe) Drumming (also called bleating or winnowing) is a sound produced by snipe as part of their courtship display flights.Van Casteren, A, et al. “Sonation in the Male Common Snipe (Capella Gallinago Gallinago L.) Is Achieved by a Flag-like Flutterin ...
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Pinioning Pinioning is the act of surgically removing one pinion joint, the joint of a bird's wing farthest from the body, to prevent flight. Pinioning is often done to waterfowl and poultry. It is not typically done to companion bird species such as parrot ...
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Plumage Plumage ( "feather") is a layer of feathers that covers a bird and the pattern, colour, and arrangement of those feathers. The pattern and colours of plumage differ between species and subspecies and may vary with age classes. Within species, ...
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Delayed feathering in chickens Delayed-feathering in chickens is a genetically determined delay in the first weeks of feather growing, which occurs normally among the chicks of many chicken breeds and no longer manifests itself once the chicken completes adult plumage. The di ...


Notes


References

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External links


Wing Feathers—US Fish and Wildlife Service document
Contains excellent photographic examples of emargination and notching in raptor remiges.
Video of feeding Magellanic woodpecker (''Campephilus magellanicus'')
Shows use of rectrices for bracing.
Video of singing male superb lyrebird (''Menuta novaehollandiae'')
Shows long modified rectrices which are used in display (though the video doesn't show full display).
Video of male club-winged manakin (''Machaeropterus deliciosus'')
Shows use of secondary remiges to produce sound.
Cornell Laboratory of Ornithology's American woodcock ''(Scolopax minor)'' recordings
#94216 has a good example of the sounds made by remiges during courtship display flight, starting at about 2:32.
Sound made by rectrices in courtship flight of common snipe ''(Gallinago gallinago)''
{{Featured article Birds Feathers Bird flight