Flight feather

Flight feathers (Pennae volatus) 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 (/ˈrɛmɪdʒiːz/), singular remex (/ˈriːmɛks/), while those on the tail are called rectrices (/ˈrɛktrɪsiːz/ or /rɛkˈtraɪsiːz/), singular rectrix (/ˈrɛktrɪks/). 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 (and therefore hunt more successfully), 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 (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 (from the Latin for "oarsman") are located on the posterior side of the wing. Ligaments 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 between the two wings, matching to a large extent in size and shape (except in the case of mutation or damage), though not necessarily in the pattern. They are given different names depending on their position along the wing.

Primaries are connected to the manus (the bird's "hand", composed of carpometacarpus and phalanges); 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, 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, 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 nine and 11, but grebes, storks and flamingos have 12, and ostriches have 16. While most modern passerines 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 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 (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 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 (those that do have the fifth secondary are said to be eutaxic). 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.