Direct and indirect assessment of parasitism risk by a cuckoo host

When the risk of encountering enemies varies in space or in time, this may select for plasticity of costly defenses. Hosts are known to vary both mobbing of adult cuckoos and egg rejection with spatiotemporal variation in brood parasitism, but it is unclear what parasitism cues they use to guide their defense plasticity. There is evidence that hosts use cuckoo activity near their nests as a direct cue, but cuckoos are secretive and resemble dangerous birds of prey so hosts may use indirect environmental predictors of parasitism too, such as their nest’s proximity to potential cuckoo lookout perches. Here, we compared reed warbler Acrocephalus scirpaceus nest defense responses with models of various enemies at a parasitized site and at a site where no common cuckoos Cuculus canorus were present. Reed warblers approached model cuckoos less closely and mobbed them less at the unparasitized site. However, at both sites, the warblers reduced their mobbing in a similar manner with increasing distance to the nearest potential cuckoo perch. The variation in response was specific to cuckoos and was not shown to harmless controls. Thus, hosts use both direct (cuckoo presence) and indirect cues (perch distance) of parasitism risk for modulating their costly defenses against their secretive parasite. We suggest that reciprocal selection for detection and suppression of direct and indirect cues provides a unifying feature of cuckoo–host arms races.