Evolution of bird eggs in the absence of cuckoo parasitism

Historical introductions of species into new habitats can create rare opportunities to test evolutionary hypotheses, such as the role of natural selection in maintaining traits. This study examines two independent introductions of the African village weaverbird (Ploceus cucullatus) to islands where selection on egg appearance traits is expected to differ markedly from that of the source populations. The color and spotting of village weaver eggs in Africa are highly consistent within clutches, but highly variable between individuals. These two features may be an evolutionary response to brood parasitism. In Africa, weavers are parasitized by each other and by the diederik cuckoo (Chrysococcyx caprius), an egg mimic. African village weavers were introduced one century ago to Mauritius, and over two centuries ago to Hispaniola. Both islands are devoid of egg-mimicking brood parasites. In these two populations, between-individual variation and within-clutch consistency in egg appearance have both decreased, as has the incidence of spotting, relative to the source populations in Africa. These reductions are more pronounced on Hispaniola, the earlier introduction. Such changes support the hypothesis that egg appearance in the African village weaver has been maintained by natural selection as a counteradaptation to cuckoo brood parasitism. These results illustrate that the removal of an agent of selection can sometimes bring about rapid evolutionary consequences.