Abstract
Information on potential hosts is essential for carrying out co-evolutionary research in cuckoo-host relationship. However, detailed information is available only for a few hosts because it is difficult to find parasitized nests thoroughly in the field. We found one parasitized nest each for the Asian Stubtail Urosphena squameiceps and the Eastern Crowned Warbler Phylloscopus coronatus in western Hokkaido. The Asian Stubtail was previously not known as a host species in Hokkaido. Although these two reddish parasitic eggs remained unhatched, it is most likely that the eggs were of the Oriental Cuckoo Cuculus optatus . One out of 67 nests of the Asian Stubtail was parasitized in this site in a total of eight years. On the other hand, five nests of the Eastern Crowned Warbler were found and three of them were parasitized in a total of four years. In this area, therefore, the Eastern Crowned Warbler is the main host and the Asian Stubtail may be a rare one for the Oriental Cuckoo. It is known that the main host of the cuckoo is the Japanese Bush Warbler Cettia diphone in central Hokkaido, however, the Oriental Cuckoo utilizes other host species in this area because of the low density of the Japanese Bush Warbler. Whatever the case, all the Oriental Cuckoo eggs found in Hokkaido seem to mimic the reddish eggs of Japanese Bush Warblers and not of other species.
