Geographical variation in the calls of the Choughs

The calls of 22 populations of the Red-billed Chough (Pyrrhocorax pyrrhocor ax) and the Alpine Chough (P. graculus) were sampled throughout the Palearc tic region. In both species calls differed in frequencies, either by latitu de or longitude in the Red-billed Chough and mostly by longitude in the Alp ine Chough. Frequencies varied according to body size, being higher in smal ler-bodied populations and lower in larger-bodied ones, as predicted by the inverse relationship between body size and frequency. We hypothesize that, besides the effect of physical constraints determining frequencies, past c limatic events and the geomorphological history of the Pleistocene also hav e influenced the present pattern of diversification. We also hypothesize th at gene flow and 'cultural' flow have constrained the evolution of clear-cu t population differences between European and North African populations, wh ose segregation could be quite recent.