Breeding productivity, phenology and habitat use of two co-occurring aerial insectivores

Aerial insectivores have been declining across North America for several decades, but the main causes of, and reasons for geographical variation in, these declines remains unclear. I directly compared variation in breeding phenology and productivity in co-occurring populations of Barn Swallows (Hirundo rustica) and Tree Swallows (Tachycineta bicolor), and the post-fledging survival and habitat use of Barn Swallows, in the lower mainland, BC, Canada. Barn Swallows had smaller clutches, and later average lay dates, but both species responded similarly to warmer spring temperatures, and interspecies differences were largely independent of habitat. Barn Swallows also had low post-fledgling survival rates, which could negatively impact population dynamics of the species in this region. Overall, our results indicate there are minor interspecies differences on the breeding grounds and therefore variation in species population trajectories in the lower mainland likely occur during the post-fledging stage, migration or on the wintering grounds.