# Does Seasonal Variation in Agility of Avian Prey Influence Body Size of Breeding Male Cooper's Hawks? And Comments on the Putative Influence of Avivory on Reproductive Output in Cities

## TL;DR

The study explores how the agility of bird prey affects the body size of male Cooper's Hawks and challenges assumptions about higher productivity in urban areas.

## Contribution

The paper provides new insights into the relationship between prey agility and hawk body size, and challenges assumptions about urban productivity in Cooper's Hawks.

## Key findings

- Smaller male Cooper's Hawks in British Columbia catch more agile prey compared to larger males in Wisconsin.
- There is no significant difference in nestling production between urban and rural Cooper's Hawk nests in British Columbia.
- The seasonal agility of avian prey influences the body size of breeding male Cooper's Hawks.

## Abstract

Recent studies have modeled predator–prey and niche dynamics in the avivorous, forest adapted Cooper's Hawk (Astur cooperii) to explain its recent and rapid colonization, including high reproductive output in varied, especially urban environments across North America. We contend that such research is compromised in part because of untenable assumptions and understudied aspects of inter‐population variation in the diet and foraging ecology of breeding males who predominately catch food for their young and markedly larger mates. These inadequacies are aggravated by unfounded generalizations in the literature of this species' diet, including a notable lack of accounting for the plausible link between inter‐population variation in body mass (size) of breeding males and the size and hence agility of their avian prey. Ecologists have dismissed the hypothesis that a males' smaller size and hence its increased agility enhances foraging efficiency because during the nestling stage males typically catch inexperienced, easy‐to‐catch nestlings and fledglings, prey for which agility seems unnecessary. However, our findings support this hypothesis because during the crucial and rarely studied pre‐incubation stage British Columbia and Wisconsin males predominately caught experienced adult birds. Our pre‐laying diet data also support the principle that predators match the size of their prey because smaller British Columbia hawks predominately caught smaller, more agile birds vs. those taken by larger Wisconsin counterparts. We found similar and high proportions of avian prey and no statistical difference between similar and high production of nestlings between Cooper's Hawk nests in urban and rural environments in British Columbia, and therefore no support for the little researched and oft‐suggested premise of greater productivity in urban vs. rural environments due to supposed high abundance of suitable avian prey in cities. Our findings combined with comments regarding pertinent literature clarify concepts and expand our knowledge of the diet and foraging ecology of breeding Cooper's Hawks.

Our findings regarding prey of breeding Cooper's Hawks in British Columbia and Wisconsin expand knowledge of their food habits and support the premise that seasonal agility of prey influences body size of nesting males. We deem it untenable to assume greater production of young at urban vs. rural nests based on proportion of avian prey use.