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.