Direction of approach by predators and flight initiation distance of urban and rural populations of birds

Abstract

Significant fitness costs of predation have resulted in the evolution of flight, which may be costly because not all attacks are successful. Some perceived attacks may not actually be attacks, resulting in physiological and behavioral costs of monitoring. Therefore, there should be a fitness advantage for individuals that efficiently monitor potential predators. Because prey encounter humans much more often in urban than in rural habitats, we predicted that urban individuals reduced the cost of approaches independent of whether approaches were direct or tangential. Thus, we predicted that birds in rural habitats would differ significantly in flight initiation distance (FID) between direct and tangential by a human, whereas such differences would be marginal in urban habitats, where all birds frequently encounter humans and hence may have adapted to frequent human proximity. The underlying mechanism allowing distinction between direct and tangential approach could be the speed or the acceleration of approach. If this mechanism of escape was efficient, we should expect individuals with longer FID when approached directly rather than tangentially to suffer less from predation by sparrowhawks Accipiter nisus. We collected information on 6753 FID from 62 species of birds from urban and rural habitats in Orsay (France) and Poznan (Poland) using direct and tangential approaches and found evidence consistent with these predictions. Urban populations of birds reacted similarly to direct and tangential approaches, whereas rural populations reacted more strongly to direct approaches. Susceptibility to predation by sparrowhawks depended on differences in FID between direct and tangential approaches, suggesting that birds were able to reduce the risk of predation in rural habitats where sparrowhawks are most common.