Seasonal increase in intensity of nest defence against little cuckoos by Japanese bush warblers

Avian brood parasitism reduces the reproductive success of hosts. Thus, hosts have evolved antiparasite strategies such as rejection of parasitic eggs and aggressive nest defence against brood parasites. Japanese bush warblers, Cettia diphone, do not reject the parasitic eggs of little cuckoos, Cuculus poliocephalus, because their eggs mimic warblers’ eggs. To examine whether Japanese bush warblers show nest defence behaviour against their parasite, I investigated the responses of the warblers to a little cuckoo dummy. Japanese bush warblers are residents, while little cuckoos are summer visitors; thus, the warblers start breeding earlier than the cuckoos. Accordingly, the risk of being parasitized increases as the season progresses. Therefore, I also examined whether the intensity of nest defence differed before and after cuckoo arrival. Japanese bush warblers showed remarkably aggressive responses to the cuckoo dummy, and levels of aggression were higher after the cuckoo arrived. These results suggest that Japanese bush warblers adaptively adjust their nest defence behaviour in response to increased risk of being parasitized. This study indicates that differences in migratory habits between hosts and brood parasites produce seasonal changes in antiparasite behaviour.