Breeding decision in migratory population with stochastic and deterministic cost.

Breeding decisions in migratory animal populations involve a trade-off between returning to natal grounds and delaying reproduction. Partial migration, in which some individuals migrate while others remain resident, is common across species. Migrating individuals often face ecological challenges, such as predation and environmental unpredictability, imposing significant breeding costs that affect their reproductive choices. We developed a discrete-time population model that considers both migration and the option to skip breeding. Our analysis of a deterministic model with constant breeding costs shows that the evolutionarily stable strategy (ESS) is always to breed. However, when accounting for environmental stochasticity from catastrophic events like floods or droughts, our extended model reveals that high breeding costs during such events may favor skipping breeding, improving long-term population growth. Conversely, always breeding remains beneficial under low costs. Our findings highlight the adaptive significance of reproductive decisions in the context of migration and environmental variability.