Potential host species of the Cuckoo in Japan, frequency of parasitism in central Honshu, and the process of the establishiment of a new parasitic relation with the Azure-winged Magpie Cyanopica cyana were studied both by literature and field work.
1) Twenty-eight species have been recorded as cuckoo hosts in Japan and twelve species of them are major hosts. The numbers of host species were greatest in central Honshu (20 species).
2) The mean rates of parasitism by the cuckoo on a total of six major hosts were more than 10 in Nagano Prefecture, central Honshu. The highest parasitism rate was 79.6% on a local popula-tion of the Azure-winged Magpie.
3) Dynamic changes of parasitic relations between cuckoos and the hosts occurred during the last 60 years. About 15 years ago, the Cuckoo began to parasitize the Azure-winged Magpie. The parasitism rate increased rapidly and has now expanded to almost all of the breeding range of the Magpie. Conversely, the Siberian Meadow Bunting Emberiza cioides has become a rare host.
4) The new parasitic relation with the magpie started as the result of their range overlap after their range expansions. In most regions, the cuckoo parasitism on the magpie did not start soon after their range overlap, but took about 10 to 15 years.
5) The characteristics of the cuckoo parasitism in Nagano Prefecture and the process of the establishment of new parasitic relation were discussed.