Significance: We address a debate that has spurred scientific and public discourse: whether conditions during autumn migration are contributing to the decline of the eastern monarch butterfly population. Using a multiscale modeling approach, we reveal that continental-scale landscape greenness during migration (proxy for nectar availability) and the amount of forest cover at winter sites significantly influence arrival colony sizes. We also demonstrate a significant demographic connection between summer and winter population sizes. Our results suggest that environmental factors during—and at the culmination of—autumn migration, combined with summer population size, explain a substantial portion of temporal variation in monarch population dynamics during a time frame after which other major putative sources of mortality (host plant and winter habitat loss) have lessened considerably.