Significance of a basal melanin layer to production of non-iridescent structural plumage color: evidence from an amelanotic Steller's jay(Cyanocitta stelleri)

Abstract

Non-iridescent structural plumage color is typically produced by coherent scattering of light within a matrix of keratin and air (a ;spongy layer') in feather barbs. It remains unclear what role, if any, the basal melanin layer underlying this spongy layer plays in the production of coloration. Amelanism in birds with structural color is a ;natural experiment' in which melanin pigmentation is lost, allowing us to identify the effects of the loss of melanin on structural color production. Here we use full-spectrum spectrometry, transmission electron microscopy and Fourier analysis to compare the color and nanostructure of an amelanotic Steller's jay (Cyanocitta stelleri Gmelin) feather with a normal blue Steller's jay feather. As a control, we also examined a white domestic chicken (Gallus gallus Linnaeus) feather. The pale amelanotic jay feather had a broad reflectance curve with a blue/green peak, while the typical blue feather had a typical distinct bell-shaped blue curve with a UV/violet peak. The white chicken feather had a typical white reflectance curve with no discrete peaks. Electron microscopy revealed that both the amelanotic and blue feather barbs contained well-formed spongy layers that were of the correct size and arrangement to produce their measured peak reflectance values, whereas the chicken feather had no spongy layer. The washed-out color of the amelanotic jay feather was thus most probably caused by the loss of the basal melanin layer, suggesting that melanin functions to absorb incoherently scattered white light from the feather barb thereby increasing the purity of the color produced by the spongy layer.