How cuckoos find the nests of their hosts and choose nests with respect to egg phenotype for parasitism is a long-standing puzzle that has so far not been solved. We recently developed an experimental design to shed light on this mystery by studying the egg-laying behavior of common cuckoos (Cuculus canorus) in nests of its Oriental reed warbler (Acrocephalus orientalis) host. Our results showed that common cuckoos only parasitized host nests with host activities but ignored the egg phenotypes in the nests. Furthermore, cuckoos distinguished between nest types of black-browed reed warbler (Acrocephalus bistrigiceps) and Oriential reed warbler and chose to parasitize the latter. This study provides strong evidence for host activities being a prime factor affecting cuckoo parasitism. Cuckoos must first locate the general site of host nests from activities by the host and then target the nests for parasitism. These observations reject the optimal egg-laying hypothesis stating that cuckoos are capable of choosing to lay eggs in host nests with visually matching egg phenotypes. Therefore, our studies challenge the idea that cuckoos recognize eggs that match their own.