Egg phenotype matching by cuckoos in relation to discrimination by hosts and climatic conditions

Abstract

Although parasites and their hosts often coexist in a set of environmentally differentiated populations connected by gene flow, few empirical studies have considered a role of environmental variation in shaping correlations between traits of hosts and parasites. Here, we studied for the first time the association between the frequency of adaptive parasitic common cuckoo Cuculus canorus phenotypes in terms of egg matching and level of defences exhibited by its reed warbler Acrocephalus scirpaceus hosts across seven geographically distant populations in Europe. We also explored the influence of spring climatic conditions experienced by cuckoos and hosts on cuckoo-host egg matching. We found that between-population differences in host defences against cuckoos (i.e. rejection rate) covaried with between-population differences in degree of matching. Between-population differences in host egg phenotype were associated with between-population differences in parasitism rate and spring climatic conditions, but not with host level of defences. Between-population differences in cuckoo egg phenotype covaried with between-population differences in host defences and spring climatic conditions. However, differences in host defences still explained differences in mimicry once differences in climatic conditions were controlled, suggesting that selection exerted by host defences must be strong relative to selection imposed by climatic factors on egg phenotypes.

Figure 1.

Geographical location of the seven sampled reed warbler populations parasitized by the European cuckoo along Europe (CH, Czech Republic; BV, Bavaria; RO, Romania; DK, Denmark; PO, Poland; SP, Spain; WP, Westphalia). Table shows data on cuckoo parasitism rate (%, with number of sampled nests in brackets), rejection rate of an experimentally added non-mimetic cuckoo-size egg (%, with number of sampled nests in brackets), number of clutches and cuckoo eggs in each population in which egg colour was measured with a spectrometer, degree of chromatic and achromatic matching between cuckoo and reed warbler eggs estimated with Vorobyev–Osorio model (see §2 for details), reed warbler and cuckoo EV (with number of eggs sampled in brackets), spring temperature and spring rainfall for the seven studied reed warbler populations. Owing to logistical problems, we could only measure with a spectrometer a random, but representative proportion (386 out of 501 clutches; i.e. 77%) of all the sampled nests for which we estimated cuckoo parasitism rate.