Egg recognition as antiparasitism defence in hosts does not select for laying of matching eggs in parasitic cuckoos

Recent studies have suggested that parasitic cuckoos have evolved laying behaviour resulting in matching of host and cuckoo eggs by choosing to lay eggs in host nests with host eggs that match the cuckoo eggs as an adaptation against egg recognition by the hosts. However, previous studies provided weak and indirect evidence with mixed results, leaving this question unresolved. Here, for the first time, we developed a robust methodology to provide unambiguous evidence that egg recognition in the host does not select for optimal egg matching during laying by the cuckoo. By using experiments that attracted parasitism, we showed that cuckoos, Cuculus canorus, indiscriminately laid eggs in oriental reed warbler, Acrocephalus orientalis, host nests containing real host eggs, egg-shaped models, stick models or coin models without any preference. Furthermore, cuckoos only selected to lay their eggs in nests with active hosts. These experiments provide evidence of cuckoos being indiscriminate in their choice of host nests, implying that coevolution of the egg phenotype of host and cuckoo eggs must have arisen from mechanisms other than matching of host eggs and those of the parasite.