Egg phenotype differentiation in sympatric cuckoo Cuculus canorus gentes

The brood parasitic common cuckoo Cuculus canorus consists of gentes, which typically parasitize only a single host species whose eggs they often mimic. Where multiple cuckoo gentes co‐exist in sympatry, we may expect variable but generally poorer mimicry because of host switches or inter‐gens gene flow via males if these also contribute to egg phenotypes. Here, we investigated egg trait differentiation and mimicry in three cuckoo gentes parasitizing great reed warblers Acrocephalus arundinaceus, marsh warblers Acrocephalus palustris and corn buntings Miliaria calandra breeding in close sympatry in partially overlapping habitat types. The three cuckoo gentes showed a remarkable degree of mimicry to their three host species in some but not all egg features, including egg size, a hitherto largely ignored feature of egg mimicry. Egg phenotype matching for both background and spot colours as well as for egg size has been maintained in close sympatry despite the possibility for gene flow.