Costs of Cuckoo Cuculus canorus parasitism to Reed Warblers Acrocephalus scirpaceus

We investigated the impact of Cuckoo Cuculus canorus parasitism on Reed Warbler Acrocephalus scirpaceus breeding success in the southeastern part of the Czech Republic. Of 1108 nests, 178 (16.1%) were parasitized. We compared several breeding parameters of parasitized and unparasitized Reed Warblers as well as the effects on breeding success of three responses adopted by parasitized individuals; acceptance, ejection and desertion. Unparasitized nests fledged 0.48 young per egg laid, whereas parasitized nests fledged a mean of only 0.05 young per egg laid. Among the parasitized individuals, ejectors had a breeding success of 0.29, while the corresponding value for the accepters was 0.03. Parasitized individuals may renest after deserting the nest, but only 30% of parasitized individuals deserted. We discuss the absence of nest desertion as a defence against Cuckoo parasitism based on the evolutionary lag and the equilibrium hypotheses. The frequency of Cuckoo parasitism was fairly constant throughout the breeding season, while the Reed Warbler's clutch size declined significantly during the same time, probably reflecting the costs of re-nesting for deserting individuals.