Seasonal density-dependence can select for partial migrants in migratory species

Whether, and which, individuals migrate or not is rapidly changing in many
populations. Exactly how and why environmental change alters migration
propensity is not well understood. We constructed density-dependent
structured population models to explore conditions for the coexistence of
migrants and residents. Our theoretical models were motivated by empirical
data identified via a systematic literature review. We find that the
equilibrium density in the season with the strongest density-dependence of
a strategy predicts whether the strategy will become dominant within the
population. This equilibrium density represents strategy fitness in a
seasonal environment and can be used to examine selection on migratory
behaviour. Whether partial migration can be maintained within a population
depends on where in the annual cycle density-dependence operates.
Diversified bet-hedging, where parents produce a mix of migrants and
residents, also maintains partial migration. Our study disentangles
density-dependent and density-independent rates in a population with
seasonal structure, potentially providing routes to explain the rapid
change in migration strategies observed in many populations.