Retaliatory cuckoos and the evolution of host resistance to brood parasites

We present a dynamic model of the evolution of host resistance to avian bro od parasites, when the latter can retaliate against hosts that reject paras itic eggs. In a verbal model, Zahavi (1979, American Naturalist, 113, 157-1 59) suggested that retaliatory cuckoos might prevent the evolution of host resistance by reducing the reproductive success of rejecter hosts (i.e. by destroying their eggs or nestlings). Here we develop a model based on the a ssociation between the great spotted cuckoo, Clamator glandarius, and its m ain host, the European magpie, Pica pica, because this is the only system t hat has provided supportive evidence, to date, for the existence of retalia tory behaviour. Our aims were (1) to derive the conditions for invasion of the retaliation strategy in a nonretaliatory parasite population and (2) to investigate the consequences of retaliation for the evolution of host defe nce. If we assume a cost of discrimination for rejecter hosts in the absenc e of parasitism, and a cost paid by a retaliator for monitoring nests, our model shows cyclical dynamics. There is no evolutionarily stable strategy, and populations of both hosts and parasites will cycle indefinitely, the pe riod of the cycles depending on mutation and/or migration rate. A stable po lymorphism of accepters and rejecters occurs only when parasites are nonret aliators. The spread of retaliator parasites drives rejecter hosts to extin ction. (C) 1999 The Association for the Study of Animal Behaviour.