The morphological basis of folded-wing posture in the American kestrel, Falco sparverius.

Gross dissection and histochemical analysis of the shoulder musculature of the American kestrel, Falco sparverius, revealed that four muscles are specialized for slow contraction and may function in the postural control of the folded wing. Mm. latissimus dorsi pars cranialis, scapulohumeralis cranialis, and brachialis were found to contain greater than 95% tonic fibers, whereas M. deltoideus minor was found to possess a relatively even mix of fast-twitch and tonic muscle fibers. M. latissimus dorsi pars cranialis, M. scapulohumeralis cranialis, and M. deltoideus minor all cross the shoulder joint caudally to the articulation, and M. brachialis crosses the elbow joint on the ventral surface of the forearm. This paper suggests postural muscles have largely been ignored in studies of avian musculature, and the need to consider a variety of possible muscle functions when analyzing locomotor muscle functions.