It would be hard to find anything as dazzling as a Bluebird standing on a fence post in the early morning sun. Its brilliant blue plumage might even be said to rival the sky itself. Too bad it's just one big illusion! It's true! Bluebirds aren't really blue . . . they just look like they are!
Most bird colorations are due to pigments deposited in their feathers. A Northern Cardinal is red because of the red pigment called carotenoids. Crows are black because their feathers contain a dark pigment called melanin. In contrast, Bluebirds do not have a single molecule of blue pigment in any of their feathers. So where does that brilliant blue color come from?
The answer is that the color is not produced by a pigment, but by the structure of the feather. The top transparent layer of each blue feather is filled with minuscule pockets of air. When sunlight strikes these pockets, all of the other visible wavelengths of light are absorbed. Only blue escapes and it is scattered in all directions. This same scattering process, created by atmospheric dust particles, is also what makes the sky appear blue.
So Thoreau was right . . . Bluebirds literally do carry the sky on their backs.
Source: WBU Bird of the month: http://www.wbu.com/botm/botm_0308.html