The coevolutionary arms race between Horsfield's Bronze-Cuckoos and Superb Fairy-wrens

Brood parasitism by cuckoos imposes high reproductive costs on hosts, selecting for the evolution of host defences. Cuckoos retaliate by evolving counter-adaptations to host defences, giving rise to a coevolutionary arms race between cuckoos and their hosts. Here we review the observational and experimental evidence for a coevolutionary arms race between Horsfield's Bronze-Cuckoos (Chalcites basalis) and Superb Fairy-wrens (Malurus cyaneus). We present evidence that the arms race has reached a uniquely advanced stage in this cuckoo-host pair; mimicry of host chicks by cuckoos has evolved in response to rejection of cuckoo chicks by hosts. We discuss the escalation of the arms race between these species in relation to the evolution of defence portfolios. Defences evolved by hosts are not only dependent on attributes of the cuckoo, but are also dependent on the success of other behaviours in host-defence portfolios. Thus, Superb Fairy-wrens rely heavily on rejection of cuckoo chicks, even though it is the costliest possible line of defence, because Horsfield's Bronze-Cuckoos can usually breach the cheaper defences against parasitism mounted earlier in the breeding attempt.