Responses of an Avian Predator and Its Isopod Prey to an Acanthocephalan Parasite

This is a study of the transmission ecology of an acanthocephalan parasite (Plagiorhynchus cylindraceus) in its natural intermediate host (the isopod Armadillidium vulgare) and definitive host the (the Starling, Sturnus vulgaris). It is the first study to demonstrate in both the laboratory and the field that parasites may produce behaviorally dimorphic prey populations with respect to definitive host foraging patterns. Differences in the prevalence of such parasites could influence the outcomes of a variety of field studies involving foraging because encounter rates in such situations may not uniformly reflect prey abundances. My experiments showed that infected A. vulgare do not behave like uninfected conspecifics. In laboratory tests, infected animals were found more frequently in less humid areas, on light—colored substrate, and in unsheltered areas. Infected females rested less often and moved farther. They did not develop ovaries and did not grow as rapidly as control females. Nestling Starlings in the field had a higher P. cylindraceus prevalence than that expected if parents had foraged on isopods without respect to infection. In the laboratory, adult Starlings consumed more infected than uninfected isopods when presented with the same number of each. This foraging p reference is probably because of increased encounters with infected isopods, which apparently frequent more exposed areas than do uninfected conspecifics.