Breeding time in a migratory songbird is predicted by drought severity and group size

Global climate change is altering the breeding phenology of many organisms, and one reported consequence of warmer average temperatures is earlier breeding times in migratory songbirds of north temperate latitudes. Less studied are the potential interactions between earlier breeding and social behavior in colonial species. We investigated how breeding time, as measured by colony initiation dates across the entire summer, in Cliff Swallows (Petrochelidon pyrrhonota) of southwestern Nebraska, USA, changed over a 30-year period and could be predicted by climatic variables, year, and colony size. Mean colony initiation date became earlier over the study, with variation best predicted by the extent of drought severity on the breeding grounds: colonies initiated earlier in warmer and drier years. Colony initiation dates were more asynchronous across the population in cooler and wetter years. There was no effect of climatic conditions during the nonbreeding season. Larger colonies started earlier in the year than smaller ones, probably because of the cost of ectoparasitism and the benefit of social foraging, both of which varied with colony size, date, and climatic conditions. The inverse relationship between breeding time and colony size was more pronounced in years with more severe drought. This study is one of the few to show that breeding phenology of a long-distance migrant bird is sensitive primarily to drought severity on the breeding grounds and that climate change can influence social behavior. If climate change exacerbates drought in the future, Cliff Swallow breeding time will likely become more strongly linked to group size.