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Birds Of A Feather Display Only A Fraction Of Possible Colors

June 23, 2011

Research reveals plumages exhibit less than a third of possible colors birds can see

Contrary to our human perception of bird coloration as extraordinarily diverse, a new study reports that bird plumages exhibit only a small fraction (less than a third) of the possible colors birds can observe.

Early lineages of living birds probably produced an even smaller range of colors, but the evolution of innovative pigments and structural (or optical) colors has allowed many birds to create more diverse and colorful plumages over time.

Researchers from the University of Cambridge and Yale University applied a mathematical model of bird vision to estimate the full range ““ or gamut ““ of avian plumage coloration and to explore how feather colors changed over the course of evolution.

Mary Caswell Stoddard of the University of Cambridge’s Department of Zoology explained: “Birds are among the most colorful organisms on the planet. To our human eyes, birds seem to possess almost every color imaginable ““ but birds see the world very differently than we do.”

Birds have an additional color cone in their retinas that is sensitive to ultraviolet light, which allows them to see many colors invisible to humans.

Stoddard and co-author Richard Prum of Yale University measured hundreds of plumage colors and analyzed them in a tetrachromatic color space, which combines raw information about the light feathers reflect with details about the colors birds can see. They found that bird plumage colors fall far short of filling the color space, leaving vast regions unoccupied.

“Just as a newspaper can only print a fraction of the colors we humans can see, bird feathers can only produce a subset of colors that are theoretically visible to other birds,” said Stoddard. “The intriguing part is thinking about why plumage colors are confined to this subset. Out-of-gamut colors may be impossible to make with available mechanisms, or they may be disadvantageous.”

Over evolutionary time, novel coloration mechanisms have evolved in different groups of birds, allowing their plumages to become more colorful.

Prum stated: “Evolutionary innovations in the form of new pigments and structural colors enabled birds to colonize new areas of avian color space. In the same way, human clothing was pretty drab before the invention of aniline dyes like mauve, but chemical inventions allowed clothing to become more diverse in color. Our study documents the history of mechanistic constraints on bird color diversity.”

Bird plumages may only represent a fraction of bird-visible colors, but how colorful are they compared to other objects in the natural world? For comparison, the researchers analyzed an extensive set of flower colors as seen by birds. They determined that bird feather colors rival the diversity of plant coloration and have achieved some striking colors unattainable by flowers.

“To explore the limits and possibilities of bird coloration is a thrilling venture, and we have much yet to discover,” said Stoddard.

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