Physics, Self-Assembly, and Evolution of Avian Structural Colors

Structural colors of feathers and avian skin are an important part of the phenotype of many birds. Understanding the optical mechanisms that produce them and the developmental mechanisms by which they are grown is essential to their functions in the lives of birds and their evolutionary history. Avian structural colors are produced by interference of light scattered from periodic nanostructures in the tissues of the skin, feathers, or iris. Although the composition of the nanostructures varies- for example, keratin and air, melanin and keratin, collagen and mucopolysaccaride, etc.– the physical mechanisms are of color production are similar. However, nanostructures with highly periodic or crystalline order will be iridescent, but quasi-ordered nanostructures which have only local, nearest neighbor order, appear very similar from all directions. Color producing biological nanostructures develop by various mechanisms of self assembly. Recent progress has been made in understanding the self assembly through phase separation (that is, chemical unmixing) of spongy medullary air and keratin nanostrutures in blue and green feather barbs. Analyses of the distributions of avian plumage colors in a tetrahedral avian color space demonstrates that structural colors provide an overwhelming large contribution to the diversity of all avian coloration (i.e. the avian plumage color gamut), indicating that structural colors have evolved to evade the physical constraints inherent in a the limited palatte of pigmentary colors.