Cuckoo parasitism on two closely-related Acrocephalus warblers in distant areas: a case of parallel coevolution?

Common Cuckoos (Cuculus canorus) parasitize nests of small passerines. The Cuckoo chicks cause the death of their nest-mates when evicting eggs or nestlings from the nests; consequently, hosts suffer from a high loss of reproduction. Host adaptations against parasitism, e.g., by egg discrimination behavior, and cuckoo counter-adaptations to hosts, e.g., by mimetic eggs, are often regarded as a result of the arms race between the two interacting species. In Hungary Great Reed Warblers (Acrocephalus arundinaceus) are the main hosts of cuckoos, suffering from heavy parasitism (ca. 40-65%). The Oriental Reed Warbler (A. orientalis), formerly a subspecies of the Great Reed Warbler (A. a. orientalis), is also a highly parasitized host in Japan (25-40%). We compared main characteristics of Cuckoo parasitism in these two distant areas from the Western and Eastern Palearctic by comparing cuckoo egg mimicry. We measured color characteristics of host and parasitic eggs by spectrophotometer. Visual modeling revealed lower chromatic distances between Cuckoo and host eggs in Hungary than in Japan, but high variation both in host and Cuckoo eggs may cause matching problems in Hungary. Achromatic (brightness) difference between host and Cuckoo eggs were lower in Japan than in Hungary, and it proved to be the most important factor affecting egg rejection. Hosts rejected Cuckoo eggs at similar frequencies (37% and 35% in Hungary and Japan, respectively). Host adaptation, i.e., egg rejection behavior, seems to be preceding Cuckoo counter-adaptations to hosts in Japan. We suggest that the Cuckoo-Great/Oriental Reed Warbler relationships developed in alternative ways in Japan and Hungary, and they represent different stages of their arms race.