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

一些鸟类拒绝孵化寄生虫在它们的巢中产卵。然而，日本灌木莺Cettia diphone并不排斥寄生的小杜鹃和小儿麻痹症的卵。林莺有可能通过时间逃避来降低寄生的成本：静止的林莺开始繁殖的时间早于前来避暑的杜鹃的到来。在日本中部伊豆群岛的三宅岛上对这一假说和其他抗寄生虫行为进行了调查。雄莺从2月中旬开始鸣叫，直到3月中旬才建立领地，似乎比研究地区的其他鸟类更早。林莺在4月初开始产卵，但在6月初小杜鹃到达后，几乎所有的巢都被寄生了。由于这种时间差异，日本灌木林莺可以在没有寄生压力的情况下饲养1-2窝幼鸟。另一方面，雏鸟的觅食季节恰逢昆虫数量的增加，这表明日本林莺的繁殖时机受到食物丰富的季节性变化的限制。林莺攻击了一只小杜鹃。寄生蜂巢的主人比非寄生蜂巢的主人攻击性小，这表明蜂巢防御是一种有效的抗寄生虫行为。在对巢址特征(如巢高、竹杆密度、能见度)的分析中，布谷鸟到达后建造的巢比杜鹃到来之前的巢更不显眼。然而，无论是能见度还是其他巢址特征都与寄生虫无关。本研究表明，幼虫和寄主之间迁移习性的差异降低了寄主的寄生成本，巢防御是一种有效的抗寄生虫策略。

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.