supracoracoideus

The supracoracoideus, or m. supracoracoideus, is a key flight muscle in birds that primarily powers the upstroke of the wing. It works in concert with the pectoralis major, which drives the downstroke, to produce the characteristic alternating wingbeats of most avian species.

Anatomy and attachments: The muscle originates from the sternum, often near the cranial aspect of the keel, and can extend onto adjacent thoracic elements. It inserts on the proximal, dorsal surface of the humerus via a tendon that passes through the shoulder through a passage known as the triosseal canal, a structure formed by the scapula, coracoid, and furcula. The tendon often acts through a pulley-like mechanism, allowing the muscle to elevate the upper wing rapidly.

Function and mechanism: When the supracoracoideus contracts, it elevates the wing for the upstroke. The distinct tendon pathway enables a rapid and powerful wing lift, a mechanical arrangement that permits quick wing elevation even as the bird’s body moves forward during flight. The downstroke is predominantly produced by the pectoralis major, which depresses the wing.

Variation and significance: Across birds, the size and prominence of the supracoracoideus varies with flight style and activity level. In many flying species it is well developed, whereas in some flightless birds it is reduced. The presence and configuration of the supracoracoideus, along with the triosseal canal anatomy, are important indicators in studies of avian flight capability in living and fossil taxa.