Data from: Causes and consequences of partial migration in a passerine bird

Many animal species have populations in which some individuals migrate and others remain on the breeding grounds. This phenomenon is called partial migration. Despite substantial theoretical work, empirical data on causes and consequences of partial migration remain scarce, mainly because of difficulties associated with tracking individuals over large spatial scales. We used stable hydrogen isotopes in claw material to determine if skylarks Alauda arvensis from a single breeding population in the Netherlands had migrated or remained resident in the previous winter and if there were causes or consequences of either strategy. Age and sex had no influence on the propensity to migrate but larger individuals were more likely to be residents. The wintering strategy was not fixed within individuals. Up to 45% of individuals measured in multiple years switched strategies. Reproductive parameters were not related to the wintering strategy, but individuals that wintered locally
experienced lower future return rates and this was directly correlated with two independent measure of immune function. Our results suggest that partial migration in skylarks is neither based on genetic dimorphism nor on an age- and sex-dependent condition. Instead the wintering strategy is related to structural size and immune status. These new insights on causes and consequences of partial migration advance our understanding of the ecology, evolution and coexistence of different life-history strategies.