Evidence suggests that structural plumage colour can be an honest signal of individual quality, but the mechanisms responsible for the variation in expression of structural coloration within a species have not been identified. We used full–spectrum spectrometry and transmission electron microscopy to investigate the effect of variation in the nanostructure of the spongy layer on expression of structural ultraviolet (UV)–blue coloration in eastern bluebird (Sialia sialis) feathers. Fourier analysis revealed that feather nanostructure was highly organized but did not accurately predict variation in hue. Within the spongy layer of feather barbs, the number of circular keratin rods significantly predicted UV–violet chroma, whereas the standard error of the diameter of these rods significantly predicted spectral saturation. These observations show that the precision of nanostructural arrangement determines some colour variation in feathers.
↵† The first two authors contributed in equal part to this work.