Responses of the Bush Warbler Cettia diphone to artificial eggs of Cuculus cuckoos in Japan

Artificial eggs of six different colours and control eggs of Bush Warblers Cettia diphone were introduced into nests of Bush Warblers, a host of both Little and Himalayan Cuckoos Cuculus poliocephalus and C. saturatw in Japan. All control (chocolate-brown) and artificial red eggs were accepted; all grey and all white eggs were rejected. The rejection rates of orange, pink and orange spots on grey eggs were 8%, 369; and 55%, respectively. Bush arblers are more likely to reject eggs the more dissimilar they are from their own. The results strengthen the possibility that the chocolate-brown eggs of Little and Himalayan Cuckoos may have evolved through the discriminative ability of Bush Warblers and their intolerance towards dissimilar eggs.