Weather conditions at different spatial and temporal scales influence avian post-breeding migration patterns on route

Migration plays a central role in many ecological and evolutionary processes. Global patterns of climatic variation are having a profound influence upon animal migration patterns. Even though regular counts of bird migrations at bottleneck sites can certainly offer insights into how natural populations of different species at different scales are responding to changes in weather conditions, they have not yet been widely used. By analysing a time series of regular counts, collected during autumn, of 126 species migrating during the daytime through one of the most important migratory bottlenecks in Western Europe, we found that an increase in temperature at the regional scale, as well as a decrease in precipitation level during the breeding period, may result not only in a decrease in the migration rate during autumn but also in a delay in the timing of fall migration. Furthermore, adverse weather conditions at the local scale interrupted bird migration through the bottleneck until favourable weather conditions returned. Importantly, temporal variation in the number of migratory birds followed a nonlinear pattern, something which might be partially due to the idiosyncratic responses of migratory species with different life-history characteristics to changes in weather conditions. Our results highlight that migration is a nonlinear, multiscale and multi-faceted process, suggesting that it will be difficult to predict the responses of idiosyncratic migratory species to the unpredictable effects of climate change.