Conditioned Aversions in a Guild of Egg Predators: Implications for Aposematism and Prey Defense Mimicry

An assemblage of egg predators in a wildlife refuge was exposed to surrogate greater sandhill crane eggs (Grus canadensis) injected with an illness-producing substance. Although composed primarily of common ravens (Corvus corax), this assemblage also included to a lesser and more poorly measured extent: black-billed magpies (Pica pica), coyotes (Canis latrans), raccoons (Procyon lotor), long-tailed weasels (Mustela frenata) and badgers (Taxidea taxus). Both attack and consumption of surrogate eggs were reduced in treatment sites compared with control sites containing uninjected eggs. Predation in control sites generally increased as predators responded to the bonanza offered to them. The data appear to reveal a novel interaction. Intact survival of surrogate eggs was not only high in raven-breeding territories where the pair of breeding ravens avoided the eggs, but enhanced by avoidance of breeding territories in the much larger nonbreeding population. Illness-based aversions appeared to powerfully alter predation among free-ranging predators. Avoidance of surrogate eggs was measured through fluctuations in availability of alternate foods, through the critical reproductive periods of all species concerned and among consumers varying in body size, and feeding behavior. This suggests that illness-induced aversions established among predators that consume toxic prey in nature could be an important factor in prey defense.