Transmission electron micrograph of a representative barbule from a dorsal feather of the nuptial plumage of male blue-black grassquits (magnification 10 000×). Black ovals are melanin granules and dark grey areas are keratin.
Comparison of the reflectance spectra of feathers from the nuptial plumage of male blue-black grassquits (black solid line, mean; black dashed lines, 95% CI) and the predicted spectra from the four models (blue, model 1; red, model 2; green, model 3; grey, model 4) based on average measurements of barbule nanostructures (n=25) at (a) normal and (b) 45° measurement geometries.
(a) Frequency distribution of the average number of melanin granules in each individual's melanin layer and (b) predicted spectra for a barbule with an average-width keratin layer measured for male blue-black grassquits, with total isolation by the melanin layer (solid line, model 2) and considering a melanin layer of thickness of one (dashed line) or two (dot-dashed line) average-sized melanin granules.
Predicted values of hue under normal incidence by thin-film optical models considering different values for the refractive index (n; solid lines) and the extinction coefficient (k; dashed lines) of (a) keratin and (b) melanin. Vertical dashed lines indicate the mean value of hue and its 95% CI (grey area), as estimated from the reflectance spectra obtained at normal incidence from feathers of male blue-black grassquits.
Nanostructural measurements of male blue-black grassquit feather barbules, obtained from TEM. (Each sample corresponds to the average of six measurements from three barbule micrographs of different feathers of the same individual.)
keratin cortex thickness (nm)
melanin layer thickness (nm)
melanin granules in layer
melanin granule diameter (nm)
Average values and estimated confidence limits for the hue of reflectance spectra obtained from feathers of male blue-black grassquits at two different measurement geometries, and for the difference in hue between those two measurements (ΔHue).
measurement geometry (n=25)
lower limit (nm)
upper limit (nm)
Predicted values of hue and hue shift with measurement geometry (ΔHue) obtained from selected models considering different values of melanin opacity. (Values in italics indicate hues that fall within the 95% CI shown in table 2.)