Do Common Whitethroats (Sylvia communis) discriminate against alien eggs?

In a coevolutionary arms race between a brood parasite and its host, both species are expected to evolve adaptations and counteradaptations, such as egg mimicry and egg discrimination. The Common Whitethroat (Sylvia communis) is a regular Common Cuckoo (Cuculus canorus) host in some parts of its breeding range. We studied egg discrimination in the Whitethroat in the south-eastern part of the Czech Republic in an area where the Whitethroat is parasitised only occasionally. To investigate responses to parasitic eggs, in 34 nests either a non-mimetic or a mimetic (conspecific) egg was added. All 21 non-mimetic eggs were ejected. Of the 13 mimetic eggs, 7 were ejected and 6 eggs were accepted. No significant differences were found in intraclutch variation in egg appearance between acceptors and rejecters. Ejected mimetic eggs, however, showed greater contrast with host eggs than did the mimetic eggs that were accepted. Our experiments show that Whitethroats are persistent rejecters of alien eggs at the study site. This fact is discussed in the light of the host selection and host-parasite coevolution hypotheses.