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.