Has the Australian Endemic Grey Falcon the Most Extreme Dietary Specialization among all Falco Species?

A clear understanding of a species' diet is crucial in understanding its spatio-temporal dynamics, and is, therefore, pertinent to conservation considerations. The diet of the Grey Falcon ( Falco hypoleucos ), a rare and threatened predator endemic to the Australian arid and semi-arid zone, is subject to diverging assertions; therefore, we studied its diet through direct observation of food ingestion during more than 17 years of fieldwork across the species' distribution. We found that Grey Falcons of all ages fed almost exclusively on a single type of food, namely, birds, and non-avian food items never constituted a substantial portion of any individual's diet. The extraordinary circumstances that were associated with the ingestion of non-avian food suggest strongly that, across its vast distribution, throughout the year, and throughout its life, the Grey Falcon feeds almost exclusively on birds. Further, we compared the diets of all Falco species and found that the dietary specialization is most extreme in the Grey Falcon, more so than even in the Taita ( F. fasciinucha ) and Peregrine Falcons ( F. peregrinus ). Based on aspects of the species' environment and relative prey availability, we offer an evolutionary explanation of the apparently unique dietary specialization of the arid-adapted Grey Falcon.