COS 92-7
Climate conditions on overwintering grounds influence the phenology and speed of spring migration for temperate migrants in the Upper Midwest
The timing of many springtime events throughout the world is shifting due to climate change. Studies have suggested that the spring arrival dates of many migratory birds have been getting earlier since the late 1930’s, but many of these results rely on observations from a limited number of sites or banding stations. Using data from Project FeederWatch, a continental-scale citizen science program focused on wintering birds, we analyzed observations from hundreds of feeder stations over a twenty-two year period (1990-2012) to capture the migratory front and speed for five temperate migratory birds. After calculating the difference between two threshold-based metrics of arrival (first and median arrival dates), we correlated both arrival timing and migration speed in relation to a suite of climate and environmental variables using a moving window analysis. This approach allowed us to capture the climate and environmental conditions on the migrant's temperate wintering grounds and their potential influence on spring migration phenology.
Results/Conclusions
We found strong support for the importance of multiple climate and ecological conditions on the first arrival times, the combinations of which varied by species, but commonly included: minimum temperature during the week immediately preceding arrival, total precipitation over the month preceding arrival, the normalized difference vegetation index of the wintering grounds, and indices of larger climate oscillations, such as the North Atlantic Oscillation. During spring migration, we found that the pace of migration proceeded more steadily over a longer period of time during years with an early first arrival. Conversely, during years of delayed spring arrival, the pace of migration was more abrupt and characterized by a reduced temporal window between the first and median arrival dates. Our findings suggest that many studies on avian migration phenology ignore critical information on the influence of overwintering climate conditions and their subsequent effects on arrival time and migration speed. We predict that migratory fronts of short-distance migratory birds will arrive earlier under projected climate change, but also that these migratory fronts will become less pronounced, with birds trickling into their breeding grounds over longer periods of time.