Avian migration timing advances linked to season-dependent population benefits.

While migratory species can adjust their migration timing in response to climate change, it remains unclear whether these shifts can insulate them from population decline. Here we compiled a global database pairing 4,351 population-level estimates of migration timing across 549 avian species with corresponding population trends and rates of climate change at breeding and non-breeding grounds, coupled with a full-annual-cycle theoretical model. Building on evidence for an accelerating advancement in bird migration over the past century (cumulatively advanced 13.86 days from 1970 to 2019), we show that rapidly advancing migration is beneficial for bird abundance during pre-breeding migration but detrimental during post-breeding migration. The net demographic benefit depends on pre-breeding advancement outpacing that during post-breeding migration. This benefit is more pronounced in large-bodied species. Our findings highlight that population benefits of rapidly shifted migratory behaviour are season dependent, underscoring the importance of full-annual-cycle perspectives for understanding and conserving migratory species under global change.