Relationship between carina size and sternum morphology in birds reflects physical constraints of body size and flight style

The avian sternum is the largest bone in the body to accommodate the large muscles required for flight. Previous analysis showed that variation in keel and sternum morphology only weakly reflected body size and flight style and implied that variation in caudal metasternum morphology was associated with expansion of bony trabeculae in some species. This study used area measurements to explore the idea that, to minimise body mass, variation in keel size was inversely correlated with variation in the shape of the metasternum. Data for sternum dimensions and area were collected from digital images of pectoral girdles and the sternum in articulation of 62 species representing 10 different orders. The keel area was expressed as a proportion of the total sternum area, and the area of the sternum from the ventral aspect occupied by bone was expressed as a proportion of the assumed maximum possible bone area. Phylogenetically controlled linear modelling explored the effects of body mass, order, sternum type and flight style. Sternum area and keel area exhibited isometric relationships with body mass which had different intercepts for each order. Proportional keel area was inversely related to body mass in some but not all orders. The proportional bone area exhibited a positive relationship with body mass and there was a significant effect of order. Proportional keel area exhibited a negative relationship with the proportional bone area. Both sternum type and flight style significantly affected proportional keel area and the proportional bone area. It was concluded that to save body mass, an increase in bone mass associated with the development of a keel has been mitigated by bone not developing (rather than extension of existing bone) in the caudal metasternum. Such patterns would also explain observed variation in sternum morphology in Mesozoic birds.