Wing upstroke and the evolution of flapping flight

Movements of the wing during upstroke in birds capable of powered flight are more complex than those of downstroke1,2,3. The m. supracoracoideus (SC) is a muscle with a highly derived morphology that is generally considered to be the primary elevator of the wing4,5,6. This muscle arises from the ventrally oriented sternum and its tendon of insertion passes craniodorsally through a special bony canal, around a bony process which deflects it laterally, to attach on the dorsal aspect of the humerus above the glenohumeral joint (Fig. 1). We studied the contractile properties of the SC in situ and related them to wing kinematics in the European starling (Sturnus vulgaris). Our findings indicate that the primary role of the SC is to impart a high-velocity rotation about the longitudinal axis of the humerus. This rapid ‘twisting’ of the humerus, coupled with limited humeral elevation, is responsible for positioning the forearm and hand so that their subsequent extension orients the outstretched wing appropriately for the following downstroke. This reinterpretation of the primary function of the SC provides insight into the selective advantage of its unique musculoskeletal organization in the evolution of powered flapping flight in birds. Figure 1 Anterolateral view of the right shoulder in the European starling (Sturnus vulgaris). The m. pectoralis has been removed, as well as all other wing and shoulder musculature to expose the m. supracoracoideus (SC) and its tendon of insertion on the dorsal aspect of the humerus. The fascicles of the SC arise from the dorsal half of the carina, the adjacent body of the sternum, and a small area on the base of the coracoclavicular membrane. The SC's bipinnate architecture limits tendon excursion but maximizes force production. Scale bar, 1.0cm. Abbreviations: SCA, scapula; HUM, humerus; COR, coracoid; TC, triosseal canal; FUR, furcula, STR, sternum.