Non-melanic schizochroism in Alberta Evening Grosbeaks, Coccothraustes vespertinus

Abstract

Evening Grosbeaks (Coccothraustes vespertinus) lacking all melanin pigments in their plumage have been reported in the last few winters at feeders across Alberta. The birds are completely brilliant yellow, except for primarily white wings and tail and dark eyes. The uniformity of the yellow color on the head of color variants is surprising considering that the black feathers of the crown of typical male Evening Grosbeaks normally contain little carotenoid pigments, about 4% the concentration of yellow pigments in the yellow feathers of the forehead (and about 17% the concentration in the greenish-olive feathers of the hindneck). The dearth of carotenoid pigments in the crown feathers of typical Evening Grosbeaks suggests that heavy deposition of melanin in those feathers somehow reduces concomitant deposition of carotenoids. Because the color change reported here is highly reminiscent of that which led to creation of the yellow form of the canary from a primarily green ancestor at the end of the seventeenth century, it may involve a homologous or functionally related gene.