We investigated the level of parasitism, rejection rates and breeding success of the Common Cuckoo Cuculus canorus parasitising two sympatric species of Acrocephalus warblers breeding in western Poland. In both hosts the frequency of Cuckoo parasitism was fairly constant throughout the breeding season. The parasitism rate (defined as the proportion of nests parasitised by the Cuckoo) was significantly higher in Reed Warblers Acrocephalus scirpaceus (12%) than in Marsh Warblers Acrocephalus palustris (4.6%). The rejection rate in Reed Warblers (7.4%) was the lowest ever reported for this species, while Marsh Warblers rejected 57.1% of Cuckoo eggs. As a result, the hatching success of the Cuckoo in Reed Warbler nests was higher than in Marsh Warbler nests (74.1% and 28.6%, respectively). The fledging success of Cuckoo chicks in the nests of the two host species was similar. Nest survival was significantly lower in parasitised nests than in non-parasitised ones. The breeding success of the Cuckoo (proportion of fledged young to the total number of eggs laid) was higher in the nests of Reed Warblers (29.6%) than in those of Marsh Warblers (7.1%), but the difference was not statistically significant. In 2001–2003 both host species were parasitised at a similar rate, but in 2004–2008 the level of parasitism dropped dramatically in Marsh Warblers. We discuss possible explanations for this phenomenon.
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