Acceptance by the splendid fairy-wren of parasitism by Horsfield's bronze-cuckoo: Further evidence for evolutionary equilibrium in brood parasitism

Horsfield's bronze-cuckoo (Chrysococcyx basalis) is one of several highly specialized Australian brood parasites that lay eggs adapted toward those of their hosts. Although parasitized at a mean rate of 20% over 17 years, one primary host, the splendid fairy-wren (Malurus splendens), does not reject the cuckoo's egg or nestling, despite evidence that this host-parasite association is one of long standing. Long-term reproductive and survival data for the splendid fairy-wren showed that (1) the least successful host females were more likely to be parasitized; (2) the most successful host females were more than able to compensate for the effects of parasitism when lifetime reproduction was considered; and (3) the parasitism had no apparent adverse effect on the survival of breeding females despite the fact that parasitized birds laid more clutches than those not parasitized. We suggest that a long coevolutionary history of brood parasitism and nest predation has favored adjustment to the host's life-history pattern, to the point where total acceptance of the cuckoo egg is now an evolutionarily stable strategy. The implications of these findings with respect to other host species are discussed. In particular, we challenge the current concept of the cuckoo-host association as a classic example of an escalating coevolutionary arms race.