Integration of skeletal traits in some passerines: impact (or the lack thereof) of body mass, phylogeny, diet and habitat.

Morphological integration of the bird skeleton is of great interest because it relates to issues of specialization, plasticity, and rate of evolutionary transformations of a skeleton as a whole and its anatomical regions. Despite growing interest, the integration and modularity of the skeleton of birds, in general, remain little studied. We evaluated the change of relative sizes and integration of shapes of skull, sternum and pelvis, and factors that influence the covariation of these regions among passerines. Results of both standard and phylogenetic reduced major axis showed that the relative lengths of the most studied skeletal traits were largely determined by body mass. The length of the skull scaled isometrically on body mass, and the lengths of both synsacrum and ilium showed positive allometry. Within the skull, beak length was positively allometric, whereas cranium length was negatively allometric with body mass. We found the presence of covariation between shapes of skull, sternum and pelvis using standard partial least squares (PLS) analysis and absence of covariation between most of these blocks using evolutionary PLS analysis on phylogenetic independent contrasts. Evolutionary integration is confirmed only for shapes of skull and pelvis (dorsal view).