Holding a wing horizontal: Roles for muscles of the pectoral girdle other than the main two flight muscles

Whilst many birds glide briefly with wings held horizontally, some species maintain this posture for extended periods during soaring. This is considered possible because of the contraction of the m. pectoralis that holds the wing in place, although albatrosses seem to have a physical shoulder lock that helps with this action. However, studies of this flight style have not considered the cranially orientated long-axis rotation of the humerus induced by the contraction of the main flight muscles that would depress the ulna and change the angle of the aerofoil downwards. This study explored whether the m. deltoideus major helps counteract this rotation. Muscle masses were collated from the literature and from dissections of birds to allow exploration of the allometry of muscle masses versus body mass. All muscles exhibited isometry with body mass, but relative to the size of the m. pectoralis, the m. deltoideus major was large but only in a few species that regularly soar or glide for long periods. By contrast, other elevator muscles were less variable among species. The presence of relatively large deltoideus major muscles in soaring species was suggestive that this muscle, since it originates on the scapula extending caudally and inserting on the dorsal humerus, may counteract humeral long-axis rotation around its longitudinal axis during contraction of the breast muscles. The results of this study are suggestive of previously unconsidered substantial roles for other muscles of the pectoral girdle and forelimb during different flight styles in birds.