Evolutionary adaptations are required by common cuckoos Cuculus canorus to match host eggs. Hosts may discriminate against alien eggs; hence, accurate matching of the parasite egg to the hosts' is essential. Egg shape is the least-studied component of egg mimicry, and it may also have other functions: an optimal egg shape is necessary for effective incubation. For this reason, cuckoo eggs may show a wide range of variations in shape to a set of host species. Here, we compare cuckoo and host eggs by using egg shape parameters in two distant areas: from the nests of great reed warblers Acrocephalus arundinaceus, robins Erithacus rubecula and marsh warblers Acrocephalus palustris in Hungary, and oriental reed warblers Acrocephalus orientalis, bull-headed shrikes Lanius bucephalus and black-faced buntings Emberiza spodocephala from Japan. Our results suggest the lack of evolutionary adaptation of different cuckoo gentes to their corresponding hosts in terms of egg shape. However, our analyses revealed that cuckoo eggs showed a geographical difference in egg shape.