Temporal escape of stationary Japanese Bush Warblers from brood parasitism by summer visiting cuckoos

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Some bird species reject eggs that were laid in their nests by brood-parasites. Japanese Bush Warblers Cettia diphone, however, do not reject eggs of parasitic Little Cuckoos Cuculus poliocephalus. There is a possibility that warblers reduce costs of parasitism by temporal escape: stationary warblers start breeding earlier than the arrival of summer-visiting cuckoos. This hypothesis and the other anti-parasite behaviors were investigated on Miyake Island in Izu Islands, central Japan.Male warblers started to sing in mid-February, and established territories until mid-March, which seemed to be earlier than other bird species in the study area. Warblers started egg-laying in early-April, but almost of the nests were parasitized once Little Cuckoos arrived in early-June. Due to this temporal difference, the Japanese Bush Warbler can rear 1-2 broods without the pressure of parasitism. On the other hand, the season of feeding nestlings coincided with the increase of insects, suggesting that the timing of breeding of Japanese Bush Warblers is restricted by the seasonal change in food abundance.Warblers attacked a stuff of the little Cuckoo. Owners of parasitized nests were less aggressive than owners of non-parasitized nests, suggesting nest defense is an effective anti-parasite behavior. In analyses of nest-site characteristics (e.g., nest height, density of bamboo stems, visibility), nests that were built after the cuckoo arrival were less visible than nests before the arrival. However, either visibility or other nest-site characteristics was not related to parasitism.This study shows that difference in migratory habits between a brood-parasite and the host reduces the costs of parasitism for the host, and that nest-defense is an effective anti-parasite tactic.