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 whether skylarks Alauda arvensis from a single breeding population in the Netherlands had migrated or remained resident in the previous winter and investigated whether 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 measures of immune function. Our results suggest that partial migration in skylarks is based neither on genetic dimorphism nor on an age- and sex-dependent condition. Instead, the wintering strategy is related to structural size and immune function. These new insights on causes and consequences of partial migration advance our understanding of the ecology, evolution, and coexistence of different life-history strategies. This data package contains one dataset: -data Hegemann et al Partial migration Excel table with 3 sheets. First sheet contains all raw data needed for Post-winter comparisons including all reproductive data, physiological data, morphological data and return rate as well as stable isotope values used for assigning birds as migrants, residents or unknown, respectively. From these table also comparisons between age and sex classes, consistency versus switching of strategies and tarsus length among switchers, obligate residents and obligate migrants can be extracted. Sheet 2 contains all data for the within-individual comparisons (longitudinal data). Sheet 3 contains data regarding the test for assortative mating