Identified best time windows for each pixel and each environmental variable influence on roosting phenology

Migratory birds are under threat by climate change. Successfully conserving them, requires knowing which climatic factors drive changes in their migratory behavior. Assessing the influence of weather on migratory bird is challenging, however, due to the temporal and spatial uncertainties in breeding, roosting, and wintering grounds. Weather may affect these temporally disjointed life stages separately and through carry-over effects, but they are often not studied together. Coupling migratory bird movements estimated using weather radar (NEXRAD) with long-term and large-scale environmental data, allows us to overcome these spatiotemporal uncertainties. Here, we assess environmental drivers of the phenology of post-breeding roosting of aerial insectivores in the Great Lakes region (USA) by evaluating predictors during the months leading up to roosting across species’ ranges. We conduct a spatially-explicit time-window analysis to examine the effects of 17 gridded weather and vegetation variables on swallow peak roosting phenology in the Great Lakes, which has advanced by 4.5 days over the past two decades. Our analysis spans a 21-year (2000-2020) time series and covers areas across Canada and the northern United States. We found that peak roosting timing is paced by both local conditions at the Great Lakes and distant conditions at the likely breeding and stopover sites, with warmer temperature advancing, headwind delaying, and high precipitation advancing the phenophases. Time windows selected for the possible breeding and stopover sites are mostly before or around the time of roosting, with one exception during wintertime. Our results fill in some of the remaining knowledge gaps in swallows’ migration ecology and, more generally, in migratory birds’ responses to ongoing climate change.