The reciprocal interactions between brood parasites and their hosts provide models for studying coevolution. For example, where hosts have evolved egg or chick discrimination, brood parasites have evolved mimicry of host eggs or chicks. Here, we suggest that there is another form of mimicry by cuckoos. A previous study has shown that naive small birds, with no evolutionary history of brood parasitism, are as afraid of adult common cuckoos Cuculus canorus as of sparrowhawks Accipiter nisus because of their physical resemblance. However, it has yet to be shown whether host species regard cuckoos as hawk like, or how hawk resemblance might benefit the cuckoo. We provide the first evidence that hawk resemblance involving barred underparts is an adaptive brood parasitic trait. We show by plumage manipulations of taxidermic models that reed warbler (Acrocephalus scirpaceus) hosts are more reluctant to approach and mob common cuckoos with barred rather than unbarred underparts. Our results indicate that reed warblers are more aggressive toward cuckoos that appear less hawk like and that hence, hawk resemblance facilitates access to host nests. Therefore, we suggest that cuckoos employ 2 forms of mimicry: To enhance parasitic laying, cuckoo adults are Batesian mimics of hawks, appearing dangerous to adult host survival, when in fact they could be safely attacked. At later stages, cuckoo eggs and chicks are aggressive mimics, appearing harmless but in fact dangerous to host reproduction. These strategies are each countered by host discrimination, providing the means for distinct coevolutionary arms races at successive stages of the host nesting cycle. Copyright 2011, Oxford University Press.