European CuckooCuculus canorusparasitism and host's rejection behaviour in a heavily parasitized Great Reed WarblerAcrocephalus arundinaceuspopulation

An unusually high frequency (64%) of European CuckooCuculus canorusparasitism was found in Great Reed WarblerAcrocephalus arundinaceusclutches in central Hungary. Sixty‐four per cent of the parasitized clutches contained one Cuckoo egg, 23% contained two, 10% had three and 3% had four. This means that 58% of the Cuckoo eggs were found in multiply parasitized clutches. In multiple parasitism the laying second Cuckoo removed an egg from the clutch randomly, so preferred neither the host eggs, nor the concurrent Cuckoo egg. Host response towards the parasitic eggs showed 66% acceptance, 12% ejection, 20% desertion and 2% egg burial. We found great variation in both the host and the parasitic egg colour and pattern. This reduces the chance that the parasitic egg's appearance matched that of the hosts’ but, in spite of this, almost perfect mimesis was found in 28% of the Cuckoo eggs. Poorly mimetic Cuckoo eggs were more frequently rejected by Great Reed Warblers than parasite eggs that were very similar to the host eggs. This high level of mimicry sometimes makes it difficult for the observer to identify the parasitic egg, especially when it is similar in size to the host eggs. It is also difficult for the host, as shown by the relatively high recognition error and ejection cost.