A synergistic combination of structural and pigmentary colour produces non-spectral colour in the purple-breasted cotinga, Cotinga cotinga (Passeriformes: Cotingidae)

Abstract

Most studies of animal coloration focus on spectral colours, which are colours evoked by single peaks within the wavelengths of visible light. It is poorly understood how non-spectral colours (those produced by a combination of reflectance peaks) are produced, despite their potential significance to both animal communication and biomimicry. Moreover, although both pigmentary and structural colour production mechanisms have been well characterized in feathers independently, their interactions have received considerably less attention, despite their potential to broaden the available colour spectrum. Here, we investigate the colour production mechanisms of the purple feathers of the purple-breasted cotinga (Cotinga cotinga). The purple feather colour results from both the coherent scattering of light by a sphere-type nanomatrix of β-keratin and air (spongy layer) in the barbs, which produces a blue-green colour, and the selective absorption of light in the centre of the bird-visible spectrum by the methoxy-carotenoid, cotingin. This unusual combination of carotenoid and nanostructure with a central air vacuole, in the absence of melanin, is a blueprint of a synergistic way to produce a non-spectral colour that would be difficult to achieve with only a single colour production mechanism.