Antiparasite behaviour in response to experimental brood parasitism in the great reed warbler : a comparison of single and multiple parasitism

Rejection of parasitic eggs is one of the most important adaptations of avian hosts against brood parasites. Multiple brood parasitism is relatively rare in hosts of the common cuckoo (Cuculus canorus), but naturally occurs when the rate of parasitism is high. We experimentally parasitised great reed warbler (Acrocephalus arundinaceus) clutches with non-mimetic and moderately mimetic model cuckoo eggs. In the case of single parasitism, each egg type was rejected at the same rate (68%-75%), but in the case of multiple parasitism, the rejection rate significantly increased to 96%. So multiple parasitism is in some way facilitating anti-parasite behaviour in the host. We suggest that when parasitism rate reaches high levels, e.g. at the beginning of the coevolutionary arms race, multiple parasitism may be an important component of the host's adaptation to brood parasitism in general.