Heterogenous shifts in autumn raptor migration timing associated with species‐specific relationships to weather

Understanding the timing of bird migration and its environmental drivers is key to assessing how climate change affects migratory species' annual cycles. While the spring arrival of North American migratory birds has consistently advanced with climate change, shifts in the timing of autumn departure are more variable. This variation may reflect species‐ and population‐specific differences in how weather influences departure timing. To test this hypothesis, we first explored species‐specific relationships between autumn departure timing and weather throughout the annual cycle using a multi‐step Bayesian analysis pipeline combining spatially explicit seasonal occurrence information from eBird data and a sliding‐window analysis. Analyses were conducted for four raptor species—Broad‐winged Hawks, Ospreys, Swainson's Hawks and Turkey Vultures—and included four readily available weather variables: minimum daily temperature, maximum daily temperature, precipitation and snow water equivalent. Using the results of these analyses, we forecasted autumn departure timing from 2011 to 2075 and quantified shifts in timing for each species. We found that the effects of each weather variable on departure timing differed between species. The period of weather variable influence also differed between species, with several instances of carry‐over effects from weather conditions earlier in the breeding season. Forecasts indicated that shifts in autumn departure timing will differ in direction among species, supporting the hypothesis that variation in autumn departure timing is the result of species‐specific weather associations. This suggests that selection pressures during autumn departure timing are more flexible or more dependent on individual species' ecological contexts than during spring arrival, which may be shaped by more consistent selection pressures (i.e. securing breeding territories or mates via early arrival to maximize reproductive success) across species.