Bird wings are actually modified forelimbs and thus exhibit anatomically very similar to the forelimbs of mammals and reptiles. The function of bird wings differs greatly because they are mainly used for only one purpose: to fly, sometimes combined with diving. Many diving seabirds use the wings for propulsion under water. Some othe divers use only use their legs for diving (eg cormorants and divers). Penguins are birds that have given up flying completely during their evolution.
Bird wings are for the most part built out of feathers, that largely determine the shape and characteristics of a particular flight or diving style. The primaries and secondaries are attached to the different wing bones and the other featers are attached to the skin which is also an important part of the air foil.

Flight styles

There is a large variation in structure among the seabirds that spend almost all their life on the wings and those that gave up flying completely.
Albatrosses and other large petrels have a predominantly gliding flight. Auks however beat their little wings rapidly up and down to stay airborne. This is due to the fact that auks have a very high body weight relative to the wing surface (high wing loading). For wing propulsion in a high densitiy medium like water small wings are necessary but have to be moved quickly up and down in a low density medium like aire in order to supply sufficient power to remain airborne. Albatrosses, also having a relatively high wing loading, hardly dive and  have long and narrow wings to take maximum advantage of the wind during their efficient long distance gliding flight.
Penguins have given up flying because they found rich and reliable food sources in their habitat and apparently had no reason to fly. Their wings became short and stiff, like the flippers of marine mammals.

Whatever style of flight, it is always a compromise to adaptation to climate and habitat type, but also to food preferences and supply. Every adaptation implies restrictions on another level. For example, the Divers and Auks are incapable of a gliding flight of more than a few meters and Albatross wings are almost useless for diving.
The relative size and shape of bird wings depend on very many factors. This discussion falls beyond the scope of this chapter.


The wings of birds are basically of the same design as the forelimb of other vertebrates and consists of an arm and a hand section.

Arm section

1. Upper arm - humerus
2. Sesamoid bone - os sesamoides.
3. Ulna - ulna
4. Radius - Radius

Hand section (manus)

5. Wrist - radiale and ulnare
6. Metacarpal - carpometacarpus
7. Thumb - alula
8. Digits - phalanges

The number of bones in a seabird's wing may vary slightly, but in general there are ten. Sometimes eleven or nine.
The long bones humerus, radius and ulna are hollow and also play a role in the respiration system because it contains an air sac. A hollow bone with the right wall thickness is almost as strong as a massive bone and combines a high bending strength with a low weight.

Arm section

The construction of the arm section in birds is similar to the forelimb of mammals and reptiles, of course with anatomical and functional differences.


The humerus is a hollow bone with a broad head at the proximal (body) side that forms the shoulder. The shoulder joint consists of the head and a shallow bowl where cartilaginous scapula (shoulder blade), coracoid and clavicle (furcula) are connected by ligaments.


Characteristic of this construction is that the wing can be moved up, down, for- and backward. This allows a bird complex wing movements and manoeuvres. The large and small flight muscles (M. pectoralis and M. supracoracoides respectively) are attached to ridges at the head. The small flight muscles attach themselves by a tendon at the top of the upper arm through an opening between scapula and coracoid (foramen triosseum). The large flight muscles are fixed to the bottom side of the head with a tendon.
Albatrosses and petrels have to keep their wings stretch ed for long periods. It was long thought that a kind of ‘shoulder lock’ exists to minimize muscle strain. However, this 'lock' is not a property of the joint, but is probably formed by a tendonous part of the pectoral muscle that prevents the stretched wing to come above the horizontal plane. (Penniquick 1982)
At the other end of the humerus (distal side) are the condyles where radius and ulna articulate. Just before the distal end is a projection (Processus ectepicondylaris) to which a sesamoid bone is attached in some petrel species. This bone, embedded in a 'fan' of tendons (patagial fan), is in a stretched wing more or less perpendicular to the humerus and provides additional strength to the wing. Albatrosses, shearwaters and other groups have a such a bone, but fulmarine petrels (such as the Fulmar) do not (Brooks 1936). In other seabird species it is not present. It is unclear whether thias bone is derived of lost feature, but somewhere in evolution this bone appeared superfluous or useful.

The shape and length of the humerus is dependent on the flightstyle. In the typical gliders the length of the humerus has increased. Albatrosses are therefore extremely long humeri. Frigate Birds, very good gliders, however, have a bit shorter humeri because they also use an active flapping flight in pursuit of other birds .
Sea birds who use their wings underwater propulsion the humerus is short, more curved and horizontally flattened. Auks and penguins are a good example, but very illustrative is the group of Shearwaters. All shearwaters can dive, using wings as well as the legs for propulsion. The larger ‘gliders’ that make relatively short and shallow dives, the humerus is long and only slightly flattened. In the ‘divers’ among the shearwaters have shorter humeri (relative to the manus) that are heavier built and clearly flattened. This flattening has - contrary to popular belief - not much to do with aerodynamics, but the fact that a flat bone copes better with the strong forces that 'flying' in a medium with high density (water) than a round bone (Kaiser 2007).

Humeri of the Cape Verde Shearwater Calonectris edwardsii and Sooty Shearwater Ardenna grisea. The Cape Verde is a 'glider' which usually dives only short and shallow. The Sooty is a real diving specialist and can dive deep to 68 m (Schaffer et al 2006) and uses wings and legs for propulsion.[Illustration]

Radius and ulna

Hand wing plus scapular of Macaroni Penguin Eudyptes chrysolophus
(Courtesey of W. v. Gestel)

The ulna (ulna) and radius (radius) together form the forearm and are usually about as long as the humerus. The ulna is the 'heaviest' bone of the two. In gulls, these two bones are relatively slightly longer than the humerus. In petrels usually a fraction shorter and in auks significantly shorter than the humerus. This shortening of the ulna and radius is related to the diving lifestyle and the forces that the wings must withstand. Between the two bones is room for the muscles that control the hand wing. In penguins all wing bones are flattened and are in this respect reminiscent of the internal shape of whale flippers.
In the real gliders such as albatrosses ulna and radius are close to each other and leave little room for muscles: there are only a small number of tendons. The ulna shows over its whole length a series of dots or indentations at the places where the secondaries are attached to the bone.
The joint with the humerus is called a saddle joint, with a cavity in the head of ulna and radius to make a pivotal movement on the convex articular surfaces of the humerus.

Hand section

In birds the hand is very different from mammals and the number reduced to two fingers and a rudimentary phalanx. The hand increases in length compared to the armvleugel in birds that use their wings for propulsion underwater. The wing is thereby closed for half an optimal power.

Ulnare and radiale

These two small bones form the wrist and apart from some minor differences more or less the same built in most birds. Again, a reduction has occurred, because in some mammals, including humans, the number of carpal bones Cn be up to eight.


This is equivalent to the metacarpal bones in mammals, but in birds these are reduced to a single bone, with a foramen in the middle. In fact, here are two metacarpals fused into one, that of the 2nd and 3rd finger. In penguins the alula is incorporated in the carpometacarpus.


The digits or ‘fingers’ of a bird's wing are also reduced in number to three. In the embryonic stage, there is a predisposition of five or possibly six digits, which reveals something about the number of fingers in the ancestors of modern birds (Kaiser 2007).
Remaining are the 1st digit: alula or thumb wing and the 2st digit (large or index finger) and a rudimentary part of the 3rd digit (middle finger).


The alula (thumb wing) consists of one phalanx (occasionally two, sometimes even with a nail in some bird species). The alula is present in all birds except the penguins, where it is fused to the carpomatacarpus. It is a freely movable digit with a few small feathers attached. The function of the alula is not yet fully understood. Ornithologists suggest that one function among others is in the landing because it can make a change in the airflow and turbulence along the wing edge. The alula articulates to the carpometacarpus near the wrist.
There has been discussion about whether the alula is really a thumb or an index finger remaining after an evolutionary adaptation. Recent genetic research has shown that the alula is a real thumb and the other two the 'index finger' and 'middle finger'. The outer two fingers are lost (Kaiser 2007).

2nd and 3rd finger

The 2nd digit consists of a flattened first phalanx and the last phalanx.
Next to the first joint of the 2nd digit sits the rudimentary phalanx of the 3rd finger. It is functionally merged with the 2nd finger and plays no independent role in the function of the wing. In penguins it is incorporated in the phalanx of the 2nd finger.
The second, last phalanx of the 2nd finger has a groove that holds the last primary. The remaining primaries are attached to the first phalanx and the carpometacarpus.