Constrained variation in the internal architecture of avian wing bones

Extant birds exhibit remarkable ecological disparity accompanied by widespread skeletal convergence driven by functional adaptation. Investigations of morphofunctional associations with ecological factors have frequently focused on the external morphology of avian wing bones; however, the extent to which such associations also apply to the internal structure of the wing skeleton remains understudied. Here, we investigate disparity of the internal epiphyseal and diaphyseal structure of the avian humerus and ulna, and explore its correlates with ecology. Our dataset of 140 species spans extant bird diversity, and demonstrates that the internal structure of avian wing bones exhibits limited ecological signal beyond expected secondary trends related to flightlessness and marine habits. Our work instead shows that variation is primarily determined by body size, suggesting that functional constraints on internal wing bone structure imposed by flight are essentially universal across flying birds irrespective of most ecological habits and flight styles. Despite this broad lack of ecological signal, distinctive aspects of forelimb internal structure may facilitate the identification of flightless bird taxa in the fossil record.