There is widespread evidence that individuals within and among host populations are not evenly parasitized by Common Cuckoos (Cuculus canorus). We first investigated whether the song and nest size of a host species, the Great Reed Warbler (Acrocephalus arundinaceus), reveal information on parental abilities and level of defense against Common Cuckoos. Second, we analyzed whether female Common Cuckoos' preference for host nests is predicted by the degree of song expression and the nest size of the host. Earlier-breeding hosts built bigger nests, were more active singers, and had less rich syllable repertoires than late breeders. Host nestlings raised in a big nest received more feedings than those raised in a small nest. Host males that were active singers were paired with females that built bigger nests. All host pairs rejected nonmimetic artificial eggs, but those with a big nest were more prone to reject natural Common Cuckoo eggs. Thus, Great Reed Warbler pairs with a big nest were more willing to feed nestlings, but also had higher discriminatory abilities against Common Cuckoo eggs, than those with a small nest. These findings, and female Common Cuckoos' inability to capture the information provided by Great Reed Warblers' sexual signals, may explain why the females followed a simple rule of selecting the more visible host nests in the population.