Bird Wing Anatomy: Bones, Feathers and Flight Muscles in 3D

In bird wing anatomy, the wing has three segments, like your arm: the upper arm (brachium) with the humerus, the forearm (antebrachium) with the radius and ulna, and the hand (manus) with two free wrist bones, the fused carpometacarpus and three short digits. Primary flight feathers grow from the hand, secondaries from the ulna and tertials from the humerus. (Feather Atlas)

This page uses the rock pigeon as its reference bird, the same species as the bird anatomy and bird skeleton pages, which cover the organs and the whole skeleton. A pigeon has ten primaries on each wing. (Mallet-Rodrigues, 2012) Where other birds differ, the text says so. Directions: the leading edge of the spread wing faces forward (cranial) and the trailing edge backward (caudal); proximal is towards the body and distal towards the wingtip.

The bones of a bird wing are the humerus, the radius and ulna, two free wrist bones (the radiale and ulnare), the carpometacarpus and the bones of three digits. Most birds have one phalanx in the alular digit, two in the major digit and one in the minor digit, so, counting the fused carpometacarpus as one bone, each wing has ten bones. (Baumel & Witmer, 1993)

Poultry Hub and Cavitt's ornithology notes give the same plan: two free carpals, three fused metacarpals, three digits and an ulna larger than the radius that supports the secondaries. (Poultry Hub) (Cavitt) The primaries are anchored to the major metacarpal and the phalanges of the major digit. (Kondo et al., 2018)

Not every bird follows the one-two-one pattern of phalanges. In several orders the alular digit has two phalanges, the last often with a claw, as in hoatzins and turacos; ducks and geese often have a third phalanx in the major digit; and the turkey's alular digit has two phalanges. Hollow humeri vary too: the air opening of the humerus is missing in penguins, petrels and shearwaters (but not albatrosses), loons, grebes, cormorants, many diving ducks, most shorebirds and gulls, rails and many songbirds. (Baumel & Witmer, 1993) Cavitt likewise lists divers and many small passerines as lacking pneumatic bones, while large soaring birds such as albatrosses, vultures and hawks have them. (Cavitt) For every other bone of the bird, see the bird skeleton page.

A bird wing and a human arm are built from the same bones in the same order: humerus, radius and ulna, wrist bones, hand bones and fingers. They are homologous structures, inherited from a common ancestor that had forelimbs. (Understanding Evolution) The bird's version has fewer wrist and hand bones and fuses its hand into one rigid bone that carries feathers.

The human figures come from standard anatomy: eight carpals, five metacarpals, two phalanges in the thumb and three in each finger. (TeachMeAnatomy) Which of our fingers the bird's three digits match is still unresolved: in theropod dinosaurs the fossil sequence points to digits I–III, while studies of chicken embryos found II–IV. Ornithologists therefore call them the alular, major and minor digits. (Nomina Anatomica Avium) For more on homologous and analogous structures, see comparative anatomy; for the bones, muscles and nerves of your own arm, see arm anatomy; and for a wing built from the same bones in a different way, see the bat wing vs bird wing comparison.

The feathers of a bird's wing are the flight feathers (remiges) and the coverts. Primaries grow from the hand, secondaries from the ulna and tertials from the humerus; (Feather Atlas) rows of coverts overlap their bases, and the alula sits at the wrist. (BTO) In a survey covering every living bird order, all flying species had 9–11 primaries. (Kiat & O'Connor, 2024)

Sources differ on secondaries because their number grows with the length of the ulna: Kiat and O'Connor found only six in a hummingbird, the Feather Atlas gives 9–25 depending on the species, and an ornithology lab handout counts up to 32 in albatrosses. (Kiat & O'Connor) (Feather Atlas) (lab handout) Primary counts are steadier. Most groups have ten; (Feather Atlas) only birds that lost flight long ago depart from 9–11, with 16 or more in ostriches, rheas and penguins and few or none in emus, cassowaries and kiwis. (Kiat & O'Connor)

Primaries are numbered from the wrist outward (P1 to P10) and secondaries from the wrist inward, so the outermost secondary borders the innermost primary. (BTO) In captive pigeons a single primary took 21–37 days to grow, the outer ones longest. (Mallet-Rodrigues) Flight feathers are asymmetric: the trailing vane is broader than the leading one, (Poultry Hub) an asymmetry that gives the wing its curved (cambered) profile. (Kiat & O'Connor) In soaring birds such as many birds of prey, notches in the vanes of the outer primaries (emarginations) let the wingtip spread into separate fingers. (Feather Atlas)

Feathers grow only from tracts called pterylae, with bare skin (apteria) between them. (Tseng et al., 2024) The flight feathers and their coverts form the alar tract; the humeral tract covers the top of the shoulder, from the leading edge of the wing to its trailing edge, where the scapulars lie over the wing base. (lab handout) On a perched bird the lesser coverts and the alula are usually hidden under the body feathers and scapulars. (BTO) For the parts of a single feather, see feather anatomy.

Flight feathers wear out: abrasion and ultraviolet light badly degrade even sturdy wing quills after about two years, so most birds under about 1 kg replace all their flight feathers every year. Because growing feathers leave gaps that reduce flight performance, most birds do not molt while breeding. In the simplest pattern, molt starts at the innermost primary, P1, and moves outward to P9 or P10 in a single wave. Large birds that must keep flying run several waves through the primaries at once, while loons, grebes, ducks, geese and many rails drop all their flight feathers together and spend a flightless period on the water; a mute swan needs about two months to regrow its longest primary. (Rohwer et al. (2009): allometry of the duration of flight feather molt in birds, PLoS Biology (PMC).)

Pigeons follow the inner-to-outer rule only loosely. In captive rock pigeons the inner primaries (P1–P5) were replaced mostly from September to December and the outer ones (P6–P10) from January to August, and only 30% of complete wing molts followed a perfect outward sequence. An old primary usually dropped when its neighbour was about three-quarters grown. (Mallet-Rodrigues (2012): replacement and growth of primary feathers in captive rock pigeons, Zoologia.)

Molt is also symmetrical. Matching feather tracts on the left and right wings molt in step, which matters for a flying bird; in the domestic fowl the inner flight feathers molt first. (Poultry Hub: integument (feathers, pterylae and moult).)

A bird flaps its wing with two breast muscles anchored on the keel of the sternum. The large pectoralis pulls the wing down in the power stroke. The smaller supracoracoideus lies under it, yet lifts the wing: its tendon runs up through the triosseal canal at the shoulder and pulls on the top of the humerus like a rope over a pulley. (Cavitt) (Biewener, 2011)

The patagium is the fold of skin that turns a bird's arm into part of the wing surface. Its main part, the propatagium, stretches between the shoulder and the wrist and forms the wing's leading edge. Along its front edge runs the propatagial muscle, which in some species is tendinous for most of its length and is then called the propatagial tendon. (Uno & Hirasawa (2023): origin of the propatagium in non-avian dinosaurs, Zoological Letters (PMC).)

The propatagium links the joints of the wing. When the elbow straightens, the propatagial muscle and tendon straighten the wrist at the same time; when the elbow bends, the radius slides along the ulna and folds the wrist. With the elbow and wrist moving together, a bird controls its wingbeat with only small muscles in the outer wing. Ratites, the flightless ostrich group, lack the propatagial structure. (Uno & Hirasawa (2023): origin of the propatagium in non-avian dinosaurs, Zoological Letters (PMC).)

The propatagial muscle has no counterpart in other living four-limbed animals. In chicken embryos it forms from cell masses split off the deltoid, pectoralis, biceps and extensor carpi radialis; it moves the wing joints and fine-tunes the wing's profile in flight, and some flightless birds have lost it. (Uno & Hirasawa (2025): multiple developmental origins of the avian propatagial muscle, Journal of Anatomy (PMC).)

The supracoracoideus is about one-fifth of the mass of the pectoralis. Its tendon passes over the shoulder through the triosseal foramen, which acts as a pulley, so the muscle raises the wing and rotates it (supination) during the upstroke. It is the main wing elevator at slow to moderate speeds and in hovering; at higher speeds the wing is probably lifted mainly by aerodynamic forces. (Biewener (2011): muscle function in avian flight, achieving power and control, Phil. Trans. R. Soc. B (PMC).)

Measurements in pigeons fit these proportions. Tobalske and Biewener weighed single muscles of birds averaging about 562 g: each pectoralis averaged 53 g and each supracoracoideus 10 g. Their recordings support the idea that the supracoracoideus mainly twists (supinates) the humerus, although it also helps to raise the wing. (Tobalske & Biewener (2008): contractile properties of the pigeon supracoracoideus during different modes of flight, J. Exp. Biol..)

The pulley itself varies. Its name means three-bone canal, after the coracoid, scapula and furcula that meet around it, but in some birds only the coracoid and scapula form it, and in birds with a bony bridge on the coracoid, including the rock pigeon, hummingbirds, turacos, bee-eaters, hoopoes and hornbills, the coracoid alone may enclose it. (Baumel & Witmer (1993): Osteologia, in Handbook of Avian Anatomy: Nomina Anatomica Avium, 2nd ed. (PDF).)

A bird wing makes lift because it is an airfoil: a curved surface with a thick, rounded leading edge that tapers to a thin trailing edge. Air flowing over the curved upper surface exerts less pressure than air below the wing, and the difference pushes the wing up. Lift grows with airspeed, with the angle of attack and with camber. (Cavitt)

The anatomy builds that shape. In cross-section the forearm wing has the propatagium, radius, ulna and coverts at the front and the vane of a secondary feather at the back (panel A above). Tilting the wing up raises lift only to a point; at too steep an angle the air breaks away from the upper surface and the wing stalls. Birds delay stalling with slots: the alula at the wrist and gaps between the outer primaries. (Cavitt) (Ehrlich et al.) The inner wing, which moves least, makes most of the lift; the hand sweeps through a wide arc and makes most of the thrust, each primary working as a small airfoil of its own. (Cavitt)

Two numbers describe a wing. Wing loading is the bird's weight per area of wing; (Suryan et al., 2008) heavy divers such as loons have much higher wing loading than small songbirds. (Cavitt) Aspect ratio is wing length divided by width. Long, narrow wings cut the drag made by wingtip vortices and lower the energy cost of flight, which is why albatrosses and other long-distance soarers have high aspect ratios, while short, broad wings turn faster. (Walters et al., 2026) (Cavitt)

The alula is a small structure at the joint between the hand-wing and the arm-wing, made of a digit bone and two to six feathers, and it is present in all living flying birds. It has long been compared to the leading-edge slat of an aircraft wing, which keeps lift high at steep angles. (Lee et al. (2015): the function of the alula in avian flight, Scientific Reports (PMC).)

Lee and colleagues tested this in Eurasian magpies. With a full alula, the birds descended more steeply, finished their descent sooner and changed body orientation more than the same birds while the alula was molting. In a wind tunnel, magpie wings with the alula made more lift at high angles of attack (by 1.3–12.7% in one wing tested) and often stalled later. The tip of the alula sheds a small swirling vortex that pushes air down onto the wing's upper surface and stops the airflow from separating from it. (Lee et al. (2015): the function of the alula in avian flight, Scientific Reports (PMC).)

Bird wings come in a few basic shapes, each matched to a way of flying: short, rounded wings for quick take-offs and turns, long pointed wings for speed, broad slotted wings for soaring over land, and very long, narrow wings for soaring over the sea. (Ehrlich et al.)

Within one group the shape follows habitat. Hawks that soar over open country have low-aspect-ratio wings, and woodland hunters such as the sharp-shinned hawk, like owls, have even lower ratios, which helps them turn quickly among trees. (Ehrlich et al.) Narrow thermals favour lower aspect ratios that keep a bird inside the rising column. (Cavitt)