Interspecific Bergmann’s rule pattern in birds persists despite seasonal migration

Ayumi Mizuno , Erick Lundgren, Szymon Marian Drobniak , Corey T. Callaghan , Eduardo S. A. Santos , Malgorzata Lagisz , Santiago Ortega , Xi Lin, Shinichi Nakagawa 

Climatic gradients are thought to shape animal body size, with larger species often occurring in colder environments. Yet trait-climate relationships may reflect current climatic exposure as well as the evolutionary and biogeographic histories that shaped species’ traits. Bergmann’s rule illustrates this challenge, and birds provide a useful test because seasonal migration decouples body size from climatic exposure across breeding and non-breeding seasons. We asked whether migration weakens the interspecific Bergmann’s rule pattern by reducing the negative temperature-body mass slope, or whether migrants instead differ mainly in average body mass. We analysed body mass and seasonal range-climate data for over 10,000 bird species with phylogenetic linear models, comparing resident and migratory species under breeding- and non-breeding-season climate assignments. Body mass declined with temperature in both residents and migrants under both assignments. Migrants showed a slightly shallower slope, but support for a migrant-resident slope difference was limited. Instead, migrants were consistently smaller than residents after accounting for seasonal climatic exposure, and body mass was lower in migrants with larger seasonal temperature shifts. Thus, migration does not appear to erase Bergmann's rule in birds - migratory species retain a negative temperature-body mass relationship while showing a distinct shift toward smaller body size.