Non-breeding conditions induce carry-over effects on survival of migratory birds.

Identifying the processes that limit populations is a foundational objective of ecology and an urgent need for conservation. For migratory animals, researchers must study individuals throughout their annual cycles to determine how environmental conditions limit demographic rates within each period of the annual cycle and also between periods through carry-over effects and seasonal interactions.^1,2,3,4,5,6 Our poor understanding of the rates and causes of avian migration mortality⁷ hinders the identification of limiting factors and the reversal of widespread avian population declines.^8,9 Here, we implement new methods to estimate apparent survival (hereafter survival) during migration directly from automated telemetry data¹⁰ in Kirtland's Warblers (Setophaga kirtlandii) and indirectly from mark-recapture data in Black-throated Blue Warblers (S. caerulescens). Previous experimental and observational studies of our focal species and other migratory songbirds have shown strong effects of Caribbean precipitation and habitat quality on food availability,^11,12,13,14 body condition,^{12,13,14,15,16,17,18,19} migration timing,^{11,12,15,16,20,21,22,23} natal dispersal,^24,25 range dynamics,²⁶ reproductive success,^20,22,27 and annual survival.^{18,19,20,23,28,29,30,31} Building on this research, we test the hypotheses that environmental conditions during the non-breeding period affect subsequent survival during spring migration and breeding. We found that reduced precipitation and environmental productivity in the non-breeding period strongly influenced survival in both species, primarily by reducing survival during spring migration. Our results indicate that climate-driven environmental conditions can carry over to affect survival in subsequent periods and thus likely play an important role in year-round population dynamics. These lethal carry-over effects may be widespread and are likely magnified by intensifying climate change.