Increase in Numbers and Potential Phenological Adjustment of Ruby-throated Hummingbirds (Archilochus colubris) during Autumn Migration at Hawk Mountain Sanctuary, Eastern Pennsylvania, 1990-2014

Global change can affect several aspects of bird biology, including population size and migration tinting. We used count data collected during 25 years (1990-2014) at Hawk Mountain Sanctuary, a raptor migration watch-site in eastern Pennsylvania, to investigate population changes in Ruby-throated Hummingbirds (Archilochus colubris) and the tinting of their autumn migration, in light of ongoing climate change. Hummingbird numbers increased significantly from 1990-2014. The first 5%-, 50%-, 95%- and average passage dates of hummingbirds over this time indicated an earlier passage, with the first 5% passage-date shifting earlier significantly. Passage duration (number of days between 5% and 95% of the flight) remained relatively constant front 1990-2014. In light of similar shifts in timing of spring passage of tins species, our results suggest that Ruby-throated Hummingbirds may be shifting the timing of their migratory cycle.