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Hughes (1997) hypothesized that the blue coloration of Black-billed (Coccyzus erythropthalmus) and Yellow-billed (C. americanus) cuckoo eggs evolved in response to egg discrimination by their hosts. From that hypothesis, we predicted that cuckoo hosts would eject ancestral nonmimetic (white) eggs, but be more likely to accept mimetic (blue) eggs. American Robins (Turdus migratorius) and Gray Catbirds (Dumetella carolinensis) were used to test that hypothesis because they are ejecter species that lay blue eggs and have been parasitized by Coccyzus cuckoos. Hughes' egg-mimicry hypothesis was not supported because robins and catbirds did not significantly eject white cuckoo eggs more frequently than blue cuckoo eggs. Indeed, most robins accepted both egg types. Furthermore, the observational evidence for egg mimicry is not convincing because only 5 of the 15 host species that Hughes (1997) described as having eggs that “fully or nearly match” the egg color of Black-billed and Yellow-billed cuckoos actually do so.

Did Blue Eggs of Black-billed (Coccyzus erythropthalmus) and Yellow-billed (C. americanus) Cuckoos Evolve to Counter Host Discrimination?