Painting the Bunting: Carotenoids and structural elements combine to produce the feather coloration of the male Painted Bunting

Abstract

Abstract Male Painted Buntings (Passerina ciris) display at least 6 distinct plumage colors that encapsulate much of the visible light spectrum, yet the specific mechanisms responsible for generating this diversity of color have not been identified. Here, we show that metabolically derived carotenoids and nanostructures capable of producing structural color were ubiquitous across feather patches. We used digital photography, light microscopy, spectrophotometry, carotenoid extraction, and high-performance liquid chromatography to show that the resulting color of each feather patch depended on the concentration of carotenoids, melanins, and underlying feather nanostructures. For example, we found that the blue-violet head feathers contained low concentrations of ketolated carotenoids, which is not typical of blue-violet structurally colored feathers. Additionally, the red breast and orange belly feathers contained a green-tuned structural color visible after carotenoid extraction, which is not typical of feathers that contain ketolated carotenoids. Although, none of these abnormal combinations of carotenoids and structural coloration appeared to significantly impact feather color. Conversely, we found the purple rump, dark green greater coverts, and bright yellow-green mantle feather coloration resulted from the combination of high concentrations of carotenoids and the presence of structural color. For the first time, we identify the combination of red ketolated carotenoids and blue structural color as a mechanism to produce purple feather coloration. Identifying the specific mechanisms that give rise to the diversity of colors within this species will facilitate the study of the—to date—unknown signaling functions of colors produced through the combination of carotenoids and nanostructures in Painted Buntings and other songbirds.