Variation in plumage color in the family Cuculidae has become an interesting subject for model systems in the study of avian color evolution. It is notable that in certain parasitic species, such as the Common Cuckoo Cuculus canorus and Lesser Cuckoo C. poliocephalus, it is only the females that exhibit color polymorphism, with either gray or rufous morphs. However, a general lack of systematic quantitative data on color morphs means that the underlying mechanism of color morph variation remains unclear. Our studies of wild-captured male and female cuckoos (both of the above species) have shown that their plumage morphs vary not only between the sexes, but also within sexes. In addition, the extent of color variation differs between the two species. Great variations are observed in the plumage morphs of female Lesser Cuckoos from gray to rufous with various kinds of intermediate forms, whereas only the gray females are observed in common cuckoos. Our results show that the range of plumage morphs in cuckoos is much greater than previously thought. Therefore, future studies should first explore the proximate mechanism of plumage maturation, such as the pattern of individual development and the effect of genetic/environmental factors, in order to better understand the adaptive significance of color variation in cuckoos.
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