Rates of brood parasitism vary extensively among host species and populations of a single host species. In this study, we documented and compared parasitism rates of two sympatric hosts, the Oriental Reed Warbler(Acrocephalus orientalis) and the Reed Parrotbill(Paradoxornis heudei), in three populations in China. We found that the Common Cuckoo(Cuculus canorus) is the only parasite using both the Oriental Reed Warbler and Reed Parrotbill as hosts, with a parasitism rate of 22.4%-34.3% and 0%-4.6%, respectively. The multiple parasitism rates were positively correlated with local parasitism rates across three geographic populations of Oriental Reed Warbler, which implies that higher pressure of parasitism lead to higher multiple parasitism rate. Furthermore, only one phenotype of cuckoo eggs was found in the nests of these two host species. Our results lead to two conclusions:(1) The Oriental Reed Warbler should be considered the major host of Common Cuckoo in our study sites; and(2) obligate parasitism on Oriental Reed Warbler by Common Cuckoo is specialized but flexible to some extent, i.e., using Reed Parrotbill as a secondary host. Further studies focusing on egg recognition and rejection behaviour of these two host species should be conducted to test our predictions.
