An Eastern Crowned Leaf Warbler Phylloscopus coronatus nest parasitized by an Oriental Cuckoo Cuculus saturatus with a reddish egg in Hokkaido, Japan

Abstract

The distributions of the Little Cuculus poliocephalus and Oriental C. saturatus cuckoos differ throughout Japan. In Honshu, the Little Cuckoo parasitizes mainly the Japanese Bush Warbler Cettia diphone and lays reddish mimetic eggs. The Oriental Cuckoo mainly parasitizes the Eastern Crowned Leaf Warbler Phylloscopus coronatus and lays whitish eggs. However, in central Hokkaido, where no Little Cuckoos breed, Oriental Cuckoos parasitize Japanese Bush Warblers as their main hosts, in whose nests they lay reddish eggs. Anecdotal evidence suggests that they also parasitize Eastern Crowned Leaf Warblers, laying reddish eggs, but this has not been confirmed. Here, we report the first complete evidence in which an Oriental Cuckoo chick, which hatched from a non-mimetic reddish egg, was raised by Eastern Crowned Leaf Warbler hosts in Hokkaido.