Nature’s Strategy to Color “Blue”—Cases for Avian Feathers

Colors of avian feathers can be produced by pigmentation and/or structural coloration. Interestingly, avian plumage colors, where optical nanostructures are present, only produce the color “blue”. The reason why nature uses structural coloration instead of pigmentation to produce blue in the short wavelength regions of the visible spectrum is poorly understood, despite its fundamental significance to physiology and potential to spark advances in biomimetic applications. We may postulate that natural blue pigments are chemically less stable than red pigments due to their molecular complexity so that evolutionary process has led birds to use structural ways to produce diverse array of blue colors. Here, we discuss nature’s strategy to produce color “blue” with structural ways by elucidating various cases of avian feathers using optical measurement, electron microscopy, numerical modeling, and biomimicry.<br/>We show that how birds produce and enhance appearance from photonic crystal or photonic glass nanostructures by addressing ingenious biological designs. To understand the various color-showing mechanisms in avian feathers, the correlations between the avian nanostructures and colorations were analyzed quantitatively, and the avian structural colors were mimicked with synthetic pathways to verify our understanding. For the case of common pheasant, we find that the photonic crystal nanostructures in their feathers can further modulate color in combination with the surrounding thin-film-like cortex layer to produce brilliant colors from blue to green. For Eurasian jay displaying periodic array of blue and white colors in their feathers, our analysis indicates that they vary colors by controlling only the thickness of color-producing optical nanostructures using multiple scattering effects. On the other hand, Kingfisher modulates plumage colors from blue to cyan by changing channel width of optical nanostructures. All these findings suggest that birds have various efficient way to produce blue colors with structural coloration. Inspired by nature’s strategy to color “blue”, we also demonstrate biomimetic “green” fabrication methods to display brilliant blue colors using synthetic nanoparticles and controlling the conditions similar to the cases in avian optical nanostructures.<br/>This study was funded by the National Institute of Ecology through the grant number (NIE-C-2021-18) and Human Frontier Science Program through the grant number (RFP0047/2019).