How colorful are birds? Evolution of the avian plumage color gamut

The avian plumage color gamut is the complete range of plumage colors, as seen by birds themselves. We used a tetrahedral avian color stimulus space to estimate the avian plumage color gamut from a taxonomically diverse sample of 965 plumage patches from 111 avian species. Our sample represented all known types of plumage coloration mechanisms. The diversity of avian plumage colors occupies only a portion (26–30%, using violet-sensitive and ultraviolet-sensitive models, respectively) of the total available avian color space, which represents all colors birds can theoretically see and discriminate. For comparison, we also analyzed 2350 plant colors, including an expansive set of flowers. Bird plumages have evolved away from brown bark and green leaf backgrounds and have achieved some striking colors unattainable by flowers. Feather colors form discrete hue ‘‘continents,’’ leaving vast regions of avian color space unoccupied. We explore several possibilities for these unoccupied hue regions. Some plumage colors may be difficult or impossible to make (constrained by physiological and physical mechanisms), whereas others may be disadvantageous or unattractive (constrained by natural and sexual selection). The plumage gamut of early lineages of living birds was probably small and dominated by melanin-based colors. Over evolutionary time, novel coloration mechanisms allowed plumages to colonize unexplored regions of color space. Pigmentary innovations evolved to broaden the gamut of possible communication signals. Furthermore, the independent origins of structural coloration in many lineages enabled evolutionary expansions into places unreachable by pigmentary mechanisms alone.