Our eyes allow us to see three primary colors in terms of light which are red, green and blue, but birds see a different colored world. Research has revealed the retinas of some birds are sensitive to light in the ultraviolet band, and seeing a broader spectrum of light can make as big a difference as watching black and white verses color television.
A spectrophotometer scan of songbird plumage where the birds are sexually monochromatic (males and females looking identical) like the cedar waxwings, titmice, chickadees and wrens, reveals they are actually sexually dichromatic (different once you take into account the colors including ultraviolet). To the birds themselves, males and females look quite different from one another.
In laboratory tests it was found that female birds preferred males with the brightest “invisible” plumage, perhaps proving that the UV- reflecting feather colors birds saw were very important.
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M. C. Stoddard, R. O. Prum. How colorful are birds? Evolution of the avian plumage color gamut. Behavioral Ecology, 2011; DOI: http://beheco.oxfordjournals.org/content/22/5/1042.full