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

Whether individuals migrate or not is rapidly shifting in many populations. Exactly how and why environmental change alters migration propensity is not well understood. We conducted a systematic literature review to gain insights into the mechanisms that affect the propensity to migrate. Motivated by findings from our literature review, we constructed density-dependent structured population models to explore the coexistence of migrants and non-migrants, when they coexist, and when they do not. We find that carrying capacity in a seasonal environment determines the dominant type in the population. Partial migration can be maintained, or not, depending on where in the life cycle density-dependent operates. Bet-hedging also promotes partial migration. Our study disentangles density-dependence and density-independence in seasonal environments, providing insights into the coexistence of polymorphism, potentially explaining the rapid change in migration strategy observed.