Sexual Size Dimorphism of Bird-Eating Hawks: The Effect of Prey Vulnerability

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Abstract
Among predatory birds, magnitude of sexual size dimorphism increases progressively from carrion-eating and invertebrate-eating species through fish-eating and mammal-eating species to a high in bird-eating species. Analysis of raptor hunting success demonstrates that the relation between hunting success and prey type is exactly opposite the relation between prey type and degree of dimorphism: raptors that prey upon birds experience more difficulty in the successful capture of their prey than raptors that prey upon fish, reptiles or invertebrates. A new hypothesis based upon prey vulnerability is introduced to explain differences in degree of raptor dimorphism. The prey vulnerability hypothesis maintains that lower encounter rates and capture successes resulting from the highly developed predator detection and escape abilities of avian prey may lower the availability of avian prey in comparison to nonavian prey. This would result in bird-eating raptors receiving a lower net energy intake per unit time spent foraging than non-bird-eating raptors. The prey vulnerability hypothesis supplements intersexual food competition or sexual role division hypotheses by presenting an alternative mechanism based upon prey availabilities (not prey abundances) for how differences among prey types may have led to differences in degree of raptor size dimorphism.