Costs to host defence and the persistence of parasitic cuckoos.

Raising genetically unrelated young is maladaptive, yet brood parasitism is widespread in birds. In several systems, hosts can evolve near-perfect defences against the parasite (discrimination and rejection of unlike eggs), making it difficult to understand how the parasite continues to exist. This study demonstrates costs to host defences (e.g. rejection of one's own eggs) such that once the parasite goes extinct on a particular host species, defence mechanisms are selectively disadvantageous. The consequent loss of host defences, and potential for re-exploitation of the host by the parasite, can explain the continued persistence of avian brood parasites. The results provide one general explanation for coexistence of parasites and their hosts.