Flight feather
Flight feathers are the long, stiff, asymmetrically shaped, but symmetrically paired pennaceous feathers on the wings 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 and lift, thereby enabling flight. The flight feathers of some birds 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 owls to fly silently, while the extra-stiff rectrices of woodpeckers 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, 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 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 to extending the moult over a period of several years.
Remiges
Remiges are located on the posterior side of the wing. Ligaments attach the long calami 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 between the two wings, matching to a large extent in size and shape, though not necessarily in the pattern. They are given different names depending on their position along the wing.Primaries
Primaries are connected to the manus ; these are the longest and narrowest of the remiges, and they can be individually rotated. These feathers are especially important for flapping flight, as they are the principal source of thrust, 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, 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, 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.File:Haliaeetus leucocephalus7.jpg|alt=A dark bird with a light head flies towards the viewer; its wings are lifted in a shallow "v" shape, with the tips curled upwards.|left|thumb|Bald eagle in flight with primaries spread to decrease drag and improve lift
Species vary somewhat in the number of primaries they possess. The number in non-passerines generally varies between nine and eleven, but grebes, storks and flamingos have twelve, and ostriches have sixteen. While most modern passerines have ten primaries, some have only nine. Those with nine are missing the most distal primary 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. In some species, the ligaments that bind these remiges to the bone connect to small, rounded projections, known as quill knobs, on the ulna; in other species, no such knobs exist. Secondary feathers remain close together in flight 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 in hummingbirds to as many as 40 in some species of albatross. 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. In these birds, the fifth set of secondary covert feathers does not cover any remiges, possibly due to a twisting of the feather papillae during embryonic development. Loons, grebes, pelicans, hawks and eagles, cranes, sandpipers, gulls, 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 in an effort to distinguish them from other secondaries. The term humeral 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, 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, 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. 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 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 edge of the wing—function in the same way as the slats on an airplane wing, allowing the wing to achieve a higher than normal angle of attack – and thus lift – 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 of nestling hoatzins is much delayed compared to the development of these feathers in other young birds, presumably because young hoatzins are equipped with claws 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 callused-over and unusable— into adulthood.Rectrices
Rectrices 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 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 ratites, and greatly reduced in size in penguins. Many grouse species have more than 12 rectrices. In some species, the number varies among individuals. Domestic pigeons 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 processes or body structure easier, ornithologists assign a number to each flight feather. By convention, the numbers assigned to primary feathers always start with the letter P , 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 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 tenth 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 and working inwards. Tertials are also numbered ascendantly, but in this case, the numbers continue on consecutively from that given to the last secondary.
Rectrices are always numbered from the centermost pair outwards in both directions.