Deconstructing avian flight: forelimb length correlates with coracoid dimensions in birds

Abstract Avian flight is possible because of functional adaptations of the skeleton, musculature, and the integument in the form of feathers. Bird species with short wings have a greater stroke amplitude angle than bigger birds with longer wings. The m. pectoralis generates the downstroke and the m. supracoracoideus facilitates the upstroke. Striated muscle has a limited capacity for contraction so greater stroke amplitude angles could reflect a longer muscle. The length of both flight muscles must reflect the size of the underlying skeleton, i.e. the sternum and coracoid. It was hypothesised that, to increase the distance between the sternum and the shoulder, short wings would be associated with longer coracoids, and vice versa. It was predicted that, in an inter-species comparison, coracoid dimensions would exhibit negative allometry with total forelimb skeletal length but isometry with body mass. Values for coracoid length and width, total forelimb length, and body mass were collated from the literature and measured from photographs for 146 species of bird. Phylogenetically controlled analysis showed that coracoid length scaled isometrically with body mass, but coracoid width showed positive allometry. As predicted, both coracoid length and width exhibited negative allometry with total forelimb length. Order of bird was important in determining variation in coracoid dimensions. Although coracoid length will determine muscle length, variation in sternum dimensions will also affect length of breast muscles. Further studies aiming to improve our understanding of the mechanics of flight in birds need to consider the pectoral-sternum anatomy in conjunction with the associated musculature.