FIGURE 1. Forelimb skeleton and pectoral girdle of a pigeon.

a, b, Anterolateral (a) and anterior (b) views. During a steady speed glide the bird's body weight (downward grey vector shown below the chest, F_BW) is supported by aerodynamic forces of 0.5BW from each wing (upward blue vectors, F_a). F_a has a lever arm (L_a) about the shoulder approximately 11 that of the pectoralis (L_p) requiring a pectoralis force (red vector, F_p) of about 6BW to balance the elevation moment. c, Anterolateral stereo pairs showing the saddle shaped glenoid (light blue, G), which lacks a ventral shelf to support the humeral head. The acrocoracohumeral ligament (AHL), spanning from the elevated acrocoracoid process of the coracoid to the transverse sulcus on the proximal humerus, is ideally situated to prevent ventral dislocation by the pectoralis. H, humerus, Sc, scapula (green), C, coracoid (blue), F, furcula (red), St, sternum (yellow). Scale bar in b equals 1 cm.