How does white color vary in birds? (#418)
Despite white color’s prevalence in the plumage of birds, little is known about the mechanisms by which it is produced and varies. We used spectrophotometry, transmission and scanning electron microscopy and optical modeling to investigate production of white feathers in a diverse sample of birds. Brightness of white varied extensively from >100% to <30% reflectance, and some samples showed bird-visible ultraviolet peaks. As expected, no evidence of melanin or other pigments was found in any feather. Barbs of the brightest feathers contained disordered nanoscale matrices of air holes in a keratin matrix, likely enhancing incoherent scattering. Most other feathers had no clear nanostructure and contained large central air vacuoles that varied in size, both absolutely and proportionally to the barb. Barbule number was also highly variable. We discuss the contributions of these features to intraspecific variation in white color, and its implications for signaling and other functions of white feathers.