Abstract Aim In Europe, winter severity is positively correlated with longitude. We test how this climatic cline affects biogeographical patterns in Western Palearctic homeotherms. Location Eurasia, west of 60° longitude. Methods We test the effects of longitude on body size of carnivores, using cranial measurements of 2002 specimens belonging to 11 species. We test the effects of longitude on migration patterns of birds by comparing which populations of partial migrants are sedentary and which undergo winter migration. Results Carnivore body size does not vary consistently with longitude. Populations of partial migrants are more likely to be sedentary in western Europe and to migrate from eastern Europe than vice versa. Main conclusions Longitudinal patterns in climate exert a selective force on birds but do not affect carnivore size in a consistent, predictable manner. We find no support for the mechanism suggested to promote size change, namely the fasting-endurance hypothesis.