Rapid advancement of spring migration and en route adjustment of migration timing in response to weather during fall migration in Vaux's Swifts (Chaetura vauxi)

Climate change has generated earlier springs, later falls, and different weather patterns. These changes may prove challenging to migratory species if they are unable to adjust their migratory timing. We analyzed changes in migratory timing of Vaux’s Swifts (Chaetura vauxi (J.K. Townsend, 1839)) by examining first arrivals (date the first swift arrived) and peak roost occupancy (date the maximum number of swifts were observed) at migratory roosts in both spring and fall from the citizen science organization Vaux’s Happening. First arrivals and peak occupancy date in Vaux’s Swifts advanced over time from 2008 to 2017, and the timing of first arrivals advanced with an increase in local wind gust speeds. In contrast, fall migration timing did not change over time from 2008 to 2016, but higher temperatures were associated with later fall migration (both first arrival and peak roost occupancy) and higher local wind speeds were associated with earlier fall migration (peak roost occupancy only). Like many other migratory birds, Vaux’s Swifts may be tracking earlier spring phenology, and may also be altering their migratory timing in response to local weather conditions, especially during fall migration. Our results indicate that swifts may be able to adjust their migration to a changing climate, at least in the short term.