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 gr aculus from the analysis of external measurements of museum specimens colle cted throughout the Palearctic ranges of the species. By means of univariat e and multivariate techniques we tested the importance of climatic and geog raphic factors as potential agents that might have shaped Red-billed and Al pine Chough morphology, discussing the dynamics of the interplay between na tural selection, gene flow and phylogenetic constraints. Both species exhib ited concordance in character differentiation. Bergmann's rule offered a va lid explanation of Red-billed and Alpine Chough body size variation with th e 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 ta rsus) 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 va riation, whilst in the Red-billed Chough geographic distance among populati ons (a measure of geographic isolation) and altitude were also significant. In this paper we critically evaluate the validity of subspecific categorie s quoted in the literature, given that most variation appeared to be clinal .