ASSESSING MIGRATION OF RUBY-THROATED HUMMINGBIRDS (ARCHILOCHUS COLUBRIS) AT BROAD SPATIAL AND TEMPORAL SCALES

Phenological patterns in birds appear to be temperature-dependent in part, and global temperatures are undergoing change. Many studies of bird phenology are conducted at broad temporal but local spatial scales, making it difficult to assess how temperature affects bird migration across landscapes. Recently, networks of "citizen science" volunteers have emerged whose collective efforts may improve phenology studies as biases associated with such efforts are recognized and addressed. We compared mean Ruby-throated Hummingbird (Archilochus colubris) first arrival dates from Journey North (2001-2010) with data from the North American Bird Phenology Program (1880-1969). Ruby-throated Hummingbirds arrived earlier in the more recent period throughout the eastern United States; these advances, however, varied by latitude from 11.4 to 18.2 days, with less pronounced changes above 41 degrees N. Warmer winter and spring temperatures in North American breeding grounds were correlated with earlier arrivals at lower latitudes in our recent period. Surprisingly, Ruby-throated Hummingbirds arrived later at high latitudes (42-43 degrees N) during warmer winters and later at both mid- and high latitudes (38-39, 41-44 degrees N) during warmer springs, which perhaps indicates extended migratory stopovers below 40 degrees N during these years. Overall, weather variables predicted arrival dates better in the recent than in the historical period. Our results document spatial variability in how warming temperatures affect hummingbird arrivals and add credence to the hypothesis that spatial differences in arrival patterns at high versus low latitudes could exacerbate asynchrony between some birds and their food resources and modify associated ecosystem services such as pollination and insect pest suppression. Received 28 March 2012, accepted 19 October 2012.