The research will focus on various aspects of coevolution between a brood parasite, the common cuckoo (Cuculus canorus) and its important host, the great reed warbler (Acrocephalus arundinaceus). The major emphasis will be on exploring the proximate cues responsible for cuckoo host choice and the ultimate consequences of such choice for the survival and body condition of cuckoo offspring. Both observational and experimental approaches will be adopted to assess reproductive costs of parasitism for the hosts and factors affecting their antiparasitic nest defences, including possible effects of ambient light conditions and UV part of the spectrum. As various behaviours are generally supposed to have both genetic and learning components, we will explore associations between genetic variation, host age, experience, and their defence abilities against brood parasitism. Till today, no similar complex study has ever been conducted.