To clarify the influence of host aggression on the Common Cuckoo Cuculus canorus, the cuckoo's parasitic behaviour was compared between a new host, Azure-winged Magpie Cyanopica cyana, and an old host, Great Reed Warbler Acrocephalus arundinaceus. Magpies exhibited less aggression towards cuckoos than did the warblers, and hardly attacked cuckoos at the nest. Cuckoos spent longer in the vicinity of nests while parasitizing magpie nests than when parasitizing reed warbler nests; possibly because of the magpie's lower aggression. Cuckoos parasitized magpie nests later in the day than reed warblers, despite the lower aggression of the magpies. The effect of host aggression on cuckoo parasitic behaviour and the adaptive significance of parasitic behaviour are discussed.

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