High refractive index of melanin in shiny occipital feathers of a bird of paradise

Male Lawes's Parotia, a bird of paradise, use the highly directional reflection of the structurally colored, brilliant-silvery occipital feathers in their courtship display. As in other birds, the structural coloration is produced by ordered melanin pigmentation. The barbules of the Parotia's occipital feathers, with thickness ∼3 µm, contain 6–7 layers of densely packed melanin rodlets (diameter ∼0.25 µm, length ∼2 µm). The effectively ∼0.2 µm thick melanin layers separated by ∼0.2 µm thick keratin layers create a multilayer interference reflector. Reflectance measurements yielded peak wavelengths in the near-infrared at ∼1.3 µm, i.e., far outside the visible wavelength range. With the Jamin-Lebedeff interference microscopy method recently developed for pigmented media, we here determined the refractive index of the intact barbules. We thus derived the wavelength dependence of the refractive index of the barbules' melanin to be 1.7–1.8 in the visible wavelength range. Implementing the anatomical and refractive index data in an optical multilayer model, we calculated the barbules' reflectance, transmittance and absorptance spectra, thereby confirming measured spectra. Using optical measurements and modelling, scientists have described the optics behind the shiny occipital reflectors of a bird of paradise. Birds of paradise use the brilliant colours and highly directional reflection of their plumage to attract mates; these properties originate from melanin rodlets arranged in layers. From interference microscopy measurements, researchers in the UK and the Netherlands found that the refractive index of melanin in such feathers decreases gradually from about 1.8 to 1.7 in the visible wavelength range. They also found that the absorption of melanin, generally neglected in modelling studies, significantly affects the optical properties of bird feathers. By implementing a model based on measured anatomical and refractive index data, the team reproduced the feathers’ spectra. The measured melanin data will be helpful for modelling the reflectance properties of feathers with photonic structures containing melanin.