Changes in timing of autumn migration in North European songbird populations

Although studies of changes in the timing of passerine spring migration are numerous, less is known about timing of their autumn departure. We present phenological data on 22 species based on mist-netted birds caught on the Baltic island of Christiansø during autumn migration between 1976 and 1997. We used least square and quantile regression to test for changes in departure time of the last individuals and the date at which 5%, 50% and 95% of the season total remaining to be trapped (corrected for catch effort). For the entire group of 22 migrants there was no overall change in median departure (–0.0426 days year–1, P = 0.40). Testing the 12 species for which the entire migration period was included (thus excluding many long-distance migrants), we found an overall earlier departure (–0.18 days year–1, P = 0.007). Short-distance migrants tended to show earlier departure, while long-distance migrants only showed advanced departure amongst the last individuals to leave. The magnitude of change in departure time was correlated with migration distance, longer distance migrants showing the least change. Overall, timing of autumn migration was more variable between species than the unidirectional change reported in studies of spring arrival.