How much to carry? Implications of maximum load carrying capacity for prey use of urban and rural Northern Goshawks Accipiter gentilis

ABSTRACT Capsule: Compared to their rural counterparts, urban Northern Goshawks Accipiter gentilis potentially maximize their energy delivery to the nest by exploiting heavier prey species close to their maximum carrying capacity. Aims: We aimed to assess prey use of a raptor species with distinct reversed sexual size dimorphism from a perspective of physical limitations during foraging and the transportation of prey. Methods: We estimated the theoretical maximum load carrying capacity (MLCC) of fully developed Northern Goshawks based on their flight muscle mass. Additionally, we collected data on the breeding season diet of Northern Goshawks in urban and rural habitats in Germany. By linking MLCC estimates to the diet we explained the relevance of prey size from a viewpoint of load carrying capacity. Results: Estimates for the mean (± sd) additional portable loads were 684 g (± 237) for males and 971 g (± 235) for females, accounting for 96% and 84% of their body mass, respectively. Overall prey weight averaged higher for urban areas compared to rural ones, while the majority of prey items were between 200 and 500 g and below the estimated MLCC of both sexes, with the exception of single heavy species. Results suggest that prey use of Northern Goshawks during the breeding season is barely limited by prey transportability, since both sexes are physically capable of carrying the majority of prey species to their nest. Conclusions: Urban Northern Goshawks can exploit heavier, available prey species compared to their rural counterparts, allowing them to hunt larger prey closer to their MLCC. Ultimately, by maximizing their energy delivery to the nest, this presumably constitutes one key factor why Goshawks successfully colonized European cities. This study is the first to link estimates of physical limitations in prey transportation for a free-living raptor species to its prey composition in the light of colonizing urban environments.