The Swallow and the Sparrow: how agricultural intensification affects abundance, nest site selection and competitive interactions

Intensification of farming practices is a key factor in population declines of many species, including aerial insectivores. Of these species, Tree Swallow populations have been declining rapidly in Canada, likely in response to increased pesticide use (depleting insect prey) and destruction of marginal habitats (limiting cavity-nesting opportunities). Agricultural intensification may however be favourable to other species. House Sparrows for instance could benefit from abundant nesting sites (farm buildings) and food resources (grains) in intensive landscapes. Competition for nesting sites between these two species has been observed, and could be another factor in the decline of Tree Swallows. In a 400 nest-box study system embedded along a gradient of agricultural intensification of Southern Quebec, Canada, we first assessed effects of intensification on abundance of House Sparrows by analysing 5,200 min of point counts. From these results, we modeled influence of competition and habitat on Tree Swallow nest site selection. Density of sparrows and proximity to buildings reduced the nest-box occupancy of swallows. Therefore, agricultural intensification had opposite influences on these two species, directly affecting House Sparrow abundance, but indirectly exacerbating competition pressure on swallows through competitor abundance. These results provide evidence of interspecific competition between these species, highlighting the indirect role of anthropogenic alterations on agro-ecosytems and illustrating a landscape-mediated avian competition pressure that has, to our knowledge, never been documented in farmscapes.