Modeling Feather Nanostructure Reflectance

Following previous work [17 (link)], we used a one-dimensional optical model to simulate reflectance. The model first creates vertical ‘slices’ of the feather nanostructure, all with a uniform refractive index, and then calculates light reflection as a function of wavelength at each interface using a transfer matrix approach [30 (link)]. Given that slight nanostructural variation within or among feather barbules can result in large colour differences [31 (link)], we modelled reflectance spectra using the average nanostructural parameters for each feather TEM image rather than the mean value for an individual. In previous work, we modelled reflectance spectra assuming spherical air spaces within melanosomes [17 (link)]. However, given the more rectangular shape observed in the Heliodoxa species analysed here (electronic supplementary material, figure S1), we also modelled reflectance spectra assuming block-shaped air spaces [32 (link)]. For all models, we used empirical values for the wavelength-dependent refractive indices of eumelanin [33 (link)] and keratin [34 (link)].