The evolution of egg polymorphism based on strong egg discrimination by hosts of brood parasites may be a specific adaptation against avian brood parasitism. However, different cuckoo-host systems with egg polymorphism may differ in many other aspects during coevolution. We studied the interaction between the Common Cuckoo Cuculus canorus and its host, the Daurian Redstart Phoenicurus auroreus, and detected that cuckoos use an intermediate degree of egg monomorphism to parasitize contrasting dimorphic eggs laid by redstarts. However, redstarts only possess an intermediate ability of egg recognition of dissimilar egg phenotypes with a high degree of recognition error and long latency to rejection. We discuss possible explanations for these observations and suggest that the intermediate degree of egg mimicry in the cuckoo and egg recognition in the redstart may constitute an early stage of disruptive coevolution in egg phenotypes between cuckoos and hosts. Alternatively, intermediate polymorphism may be maintained as a result of an evolutionary equilibrium between hosts and parasites.