Pacific Barrow's Goldeneye refine migratory phenology in response to overwintering temperatures and annual snowmelt

Lay Summary & BULL; Migratory birds time their annual migrations to take advantage of temporal and spatial variation in resources, disease, and predation. & BULL; Timing of these migrations is broadly determined by an internal biological clock set by day length, but individuals may refine when and how they migrate in response to environmental conditions. & BULL; We used 11 years of satellite tracking data to investigate migratory cues in Barrow's Goldeneye, a species of sea duck. & BULL; We found that birds used weather conditions as migratory cues, leaving their wintering grounds earlier in warmer springs and arriving on their breeding grounds earlier when the snow melted earlier in the year. & BULL; Because individuals respond to conditions both where they start and where they end spring migration, our results suggest that Barrow's Goldeneye update their migratory decisions during migration. & BULL; Barrow's Goldeneye's ability to respond to environmental cues suggests that they may be able to adapt to ongoing climate change. Timing of seasonal bird migrations is broadly determined by internal biological clocks, which are synchronized by photoperiod, but individuals often refine their migratory timing decisions in response to external factors. Using 11 years of satellite telemetry data, we show that Pacific Barrow's Goldeneye (Bucephala islandica) at higher latitudes initiated spring and molt migrations later and fall migration earlier than individuals at lower latitudes. We further show that individuals refined migratory timing in response to interannual variation in environmental conditions. Individual Barrow's Goldeneye initiated spring migration earlier in years with warmer springs at their overwintering locations and concluded spring migration earlier in years with earlier annual snowmelt on their breeding grounds. Because individuals respond to conditions both where they initiate and where they conclude spring migration, our results suggest that Barrow's Goldeneye update their migratory decisions en route. For all 3 migrations in their annual cycle, birds delayed initiating migration if they had been captured and tagged prior to that migration. Birds that initiated migration late for their latitude were less likely to include a stopover and completed that migration faster, partially compensating for delayed departures. Our results are consistent with the hypothesis that Barrow's Goldeneye uses a combination of endogenous cues and environmental cues in migratory decision making. Sensitivity to environmental cues suggests that Barrow's Goldeneye may have behavioral plasticity that is adaptive when faced with ongoing climate change.