Data from: Qualitative skeletal correlates of wing shape in extant birds (Aves: Neoaves)

Background: Among living fliers (birds, bats, and insects), birds display relatively high aspect ratios, a dimensionless shape variable that distinguishes long and narrow vs. short and broad wings. Increasing aspect ratio results in a functional tradeoff between low induced drag (efficient cruise) and increased wing inertia (difficult takeoff). Given the wide scope of its functional effects, the pattern of aspect ratio evolution is an important factor that contributes to the substantial ecological and phylogenetic diversity of living birds. However, because the feathers that define the wingtip (and hence the wingspan and aspect ratio) often do not fossilize, resolution in the pattern of avian wing shape evolution is obscured by missing information. Here I use phylogenetic constrained ordination to investigate the relationship between skeletal proxies of flight feather attachment, aspect ratio, and body mass. The resulting model is used to infer aspect ratios for two fossil avian taxa with preserved wing feathers and the Cretaceous ornithurine bird Ichthyornis. Results: An accessory lobe of the internal index process of digit II-1, a bony correlate of distal primary attachment, shows significant relationships to aspect ratio and body mass independent of the morphology of the rest of the forelimb skeleton. The dorsal phalangeal fossae of digit II-1, which house the follicles of distal primaries VIII and IX, also show a trend of increased prominence with higher aspect ratio. Quill knobs on the ulna are examined concurrently, but do not show consistent signal with respect to wing shape. Conclusions: Although quill knobs are most frequently cited as skeletal correlates of flight performance in birds, their relationship to wing shape is inconsistent among extant taxa, and may reflect diverging selection pressures acting on a conserved architecture. In contrast, correlates of distal primary feather attachment on the major digit show convergent responses to increasing aspect ratio within Neoaves.