Ecogeographic correlates of morphometric variation in the Red-billed Chough Pyrrhocorax pyrrhocorax and the Alpine Chough Pyrrhocorax graculus

We estimated the magnitude of intraspecific geographic variation in the Red-billed Chough Pyrrhocorax pyrrhocorax and the Alpine Chough Pyrrhocorax graculus from the analysis of external measurements of museum specimens collected throughout the Palearctic ranges of the species. By means of univariate and multivariate techniques we tested the importance of climatic and geographic factors as potential agents that might have shaped Red-billed and Alpine Chough morphology, discussing the dynamics of the interplay between natural selection, gene flow and phylogenetic constraints. Both species exhibited concordance in character differentiation. Bergmann's rule offered a valid explanation of Red-billed and Alpine Chough body size variation with the largest birds being found at higher elevation, or in colder and more and regions. Shape was also important. The extremities of the body (bill and tarsus) were longer in warmer areas, in line with Allen's rule. In the Alpine Chough, temperature seemed to be the most important cause of body trait variation, whilst in the Red-billed Chough geographic distance among populations (a measure of geographic isolation) and altitude were also significant. In this paper we critically evaluate the validity of subspecific categories quoted in the literature, given that most variation appeared to be clinal.