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 dramatically 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. Finally, 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.

鸟类以其多样绚丽的羽毛（plumage）而闻名。本研究证实，唐纳雀属（Tangara）鸟类艳丽的羽毛色彩，可通过隐藏于外层彩色羽毛之下的「隐性」无色素（achromatic，即白色或黑色）羽毛层得到显著强化。
本研究通过高光谱成像（hyperspectral imaging）、多光谱摄影（multispectral photography）、显微分光光度法（microspectrophotometry）、反射分光光度法（reflectance spectrophotometry）及光学建模等技术手段，证实隐性白色与黑色羽毛层在鸟类体表呈系统性分布：它们可通过调控与色素或纳米结构（nanostructures）相互作用的背散射光（backscattered light）总量，分别提升类胡萝卜素着色（carotenoid-pigmented）羽毛与结构色（structurally colored）羽毛的亮度及饱和度。
本研究对比了雌雄个体的羽毛色彩，发现部分唐纳雀属鸟类的类胡萝卜素着色羽毛存在雌雄二色性（sexual dichromatism），其成因主要源于雄性的白色羽毛层与雌性的黑色羽毛层差异，而非类胡萝卜素着色程度的不同。
最后，本研究发现隐性白色与黑色羽毛层在色彩艳丽的雀形目鸟类（passerines）中广泛分布。隐性羽毛层可能在鸟类绚丽羽毛的演化过程中发挥了此前被忽视却至关重要的作用。