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

Recent studies have modeled predator-prey and niche dynamics in the avivorous, forest adapted Cooper's Hawk (<i>Astur cooperii</i>) 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.