Hidden white and black feather layers enhance plumage coloration in tanagers and other songbirds

Birds are renowned for their diverse and colorful plumage. Here, we demonstrate that vibrant plumage in the tanager genus Tangara is substantially intensified by a "hidden" layer of achromatic (white or black) plumage concealed beneath the outermost colorful layer. Using hyperspectral imaging, multispectral photography, microspectrophotometry, reflectance spectrophotometry, and optical modeling, we show that hidden white and black feather layers are systematically distributed on the body to enhance the brightness and saturation of carotenoid-pigmented and structurally colored plumage, respectively, by increasing or decreasing the amount of backscattered light that interacts with pigments or nanostructures. We compare male and female coloration and show that sexual dichromatism in some Tangara carotenoid plumage stems primarily from white layers in males and black layers in females rather than from differences in carotenoid pigmentation. Last, we find that white and black hidden feather layers are widespread in colorful passerines. Hidden feather layers likely play a previously overlooked but critical role in colorful plumage evolution in birds.