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The aim of the present study was to investigate the question of whether blue Cuckoo Cuculus canorus eggs have evolved as an adaptation for successful parasitism of host species which themselves lay blue eggs. For this purpose we inspected 11 870 Cuckoo eggs, 361 of which (3.04%) were blue, in host species' clutches held in European egg collections. Blue Cuckoo eggs occurred at significantly higher frequencies in the clutches of Redstarts Phoenicurus phoenicurus, Whinchats Saxicola rubetra, Wheatears Oenanthe oenanthe and Pied Flycatchers Ficedula hypoleuca than in host species which do not lay blue eggs, thus supporting our main prediction. Discrimination against non-mimetic Cuckoo eggs by Redstarts has probably been the strongest selective force behind the evolution of blue Cuckoo eggs. The Whinchat may also have played an important role. Cuckoo eggs in nests of both these species show a, statistically significant, better mimicry with host eggs than in the nests of either Pied Flycatcher or Wheatear. Pied Flycatchers and Wheatears usually nest in cavities with entrances too narrow to permit a young Cuckoo to leave. These two hosts show no discriminatory behaviour towards non-mimetic (i.e. other colours than blue) Cuckoo eggs and have therefore probably played a minor role in the evolution of blue Cuckoo eggs. Presumably they have been accidentally parasitized by "Redstart" (or "Whinchat") Cuckoos.
Journal of Avian Biology © 1995 Nordic Society Oikos