How are different Common Cuckoo Cuculus canorus egg morphs maintained? An evaluation of different hypotheses

The aim of this study was to investigate the occurrence of Common Cuckoo Cuculus canorus egg morphs in relation to host species and nest sites. The study is based on analyses of museum egg collections and field observations of Common Cuckoo parasitism in three regions of the Czech Republic. Twenty-four passerine species were recorded as Common Cuckoo hosts, and 736 Common Cuckoo eggs were classified into eleven different morphs. The most common one, the Sylvia egg morph, was found in the nests of 20 host species. Several hypotheses to explain why this morph is so common are discussed. Our results indicate that some Common Cuckoo gentes parasitise different host species with different habitat preferences. Whereas the Anthus morph was significantly more common in clutches of ground-nesting species, the Sylvia gens parasitised nests above ground level (e.g. in reeds and bushes). This may give some support to the idea that these gentes are maintained in accordance with the Nest site hypothesis and the Habitat imprinting hypothesis, which state that Common Cuckoos are not necessarily strict host specialists, but that their nest searching is guided by the general habitat structure. On the other hand, the fact that some egg morphs, like European Robin Erithacus rubecula, White/Pied Wagtail Motacilla alba/yarrellii, European Reed Warbler Acrocephalus scirpaceus and shrike Lanius spp., were mostly found in nests of the particular host species with which they correspond, supports the Host preference hypothesis, i.e. that these Common Cuckoo females specialise on only one particular host species

Honza M., Moksnes A., Roskaft E. & Stokke B.G, ARDEA 89 (2): 341-352.