The evolution of egg rejection by cuckoo hosts in Australia and Europe

Exploitation of hosts by brood parasitic cuckoos is expectedto stimulate a coevolutionary arms race of adaptations and counteradaptations.However, some hosts have not evolved defenses against parasitism.One hypothesis to explain a lack of host defenses is that thelife-history strategies of some hosts reduce the cost of parasitismto the extent that accepting parasitic eggs in the nest is evolutionarilystable. Under this hypothesis, it pays hosts to accept cuckooeggs if (1) the energetic cost of raising the cuckoo is low,(2) there is time to renest, and (3) clutch size is small. Weparasitized the nests of host and nonhost species with nonmimeticmodel eggs to test whether the evolution of egg recognitionby cuckoo hosts could be explained by life-history variablesof the host. The most significant factor explaining rates ofrejection of model eggs was whether or not a species was a cuckoohost, with hosts rejecting model eggs at a higher rate thannonhosts. Egg-rejection rates were also explained by visibilitywithin the nest and by cuckoo mass. We found little supportfor the life-history model of egg rejection. Our results suggestthat parasitism is always sufficiently costly to select forhost defenses and that the evolution of defenses may be limitedby proximate constraints such as visibility within the nest.