Variable shifts in spring and autumn migration phenology in North American songbirds associated with climate change

Monitoring studies find that the timing of spring bird migration has advanced in recent decades, especially in Europe. Results for autumn migration have been mixed. Using data from Powdermill Nature Reserve, a banding station in western Pennsylvania, USA, we report an analysis of migratory timing in 78 songbird species from 1961 to 2006. Spring migration became significantly earlier over the 46-year period, and autumn migration showed no overall change. There was much variation among species in phenological change, especially in autumn. Change in timing was unrelated to summer range (local vs. northern breeders) or the number of broods per year, but autumn migration became earlier in neotropical migrants and later in short-distance migrants. The migratory period for many species lengthened because late phases of migration remained unchanged or grew later as early phases became earlier. There was a negative correlation between spring and autumn in long-term change, and this caused dramatic adjustments in the amount of time between migrations: the intermigratory periods of 10 species increased or decreased by > 15 days. Year-to-year changes in timing were correlated with local temperature (detrended) and, in autumn, with a regional climate index (detrended North Atlantic Oscillation). These results illustrate a complex and dynamic annual cycle in songbirds, with responses to climate change differing among species and migration seasons.