Summary of VARPART and RDA results. (a) Plot of taxon RDA scores. Hue and shading of points denotes aspect ratio. Silhouettes show representative wing shapes for Anser, Cathartes, Sula, and Fregata. (b) Inset biplot representing PGLS correlations among response matrix Y (solid rays) and explanatory matrix X (dashed rays) in the canonical RDA axes. Note the strong positive correlation between aspect ratio (AR) and distal primary attachment features (iip2). drp: dorsal remigial papillae; crp: caudal remigial papillae; dpf: dorsal phalangeal fossae; iip1: presence/absence of internal index process; iip2: shape of internal index process; fp: flexor process digit III. (c) – (d) Proximal phalanges of digit II (II-1) from Rallus and Fregata in dorsal view, show morphological disparity at the extremes of the first RDA axis. (e) Representative tree topology with an ancestral character state reconstruction of AR—hues and shading correspond to (a).