Biometric differentiation of breeding and non-breeding Song Thrushes at the southwestern limit of their distributional range

ABSTRACT The migratory behaviour of European Song Thrush (Turdus philomelos) populations ranges from resident in the south and west to partly or totally migratory in northern, central and eastern Europe. This varied migratory behaviour could be reflected in morphological adaptations, with migrants having a proportionally longer wing, lower tail:wing ratio, and, if foraging more on fruits, deeper and wider bills. We used data from Song Thrushes captured during the breeding season and in winter in northern Iberia to test for morphological variations between these two periods. In three consecutive years (2014–16), Song Thrushes were captured with mist nests in a region of southern Navarra (Spain) during the breeding season (from May to July) or in the winter period (October to February), during a period of four hours starting at dawn per sampling day. We consider the winter population to be composed of a high proportion of visitors. Breeding Song Thrushes showed proportionally longer tails for given wing lengths, even though wing length did not vary between the two periods. Moreover, winter visitors showed narrower and less deep bills than breeding Song Thrushes. Potential explanations for these results are discussed.