Although no blue pigment has been found in any vertebrate, blue is by no means an uncommon color among the higher animals. ---Hazel Rossoti in Colour: Why the World isnt Grey (Princeton University Press, Princeton NJ, 1983) pg. 94.
By a change of light, according as the beams strike it vertically or aslant, ...a peacock's tail, profusely illumined, changes color as it is turned this way or that. These colors, then, are created by a particular incidence of light. Hence, no light, no color.---Lucretius (50 BCE).
What makes the blue color in this peacock feather? Any thoughts? I looked at one of the feather barbs under the light transmission microscope in my laboratory. The attached barbules were transparent and beige, except when they happened by chance to cross over one another. In the region where there was overlap, I saw an iridescent blue coloration. If I moved the barbules so there was no overlap, there was no color. Does the peacock feather have a photonic band gap in the blue? What about an embedded crystal structure on the order of the wavelength of blue light*? See how the "speckle" applet preferentially directs waves from two dimensional crystals in certain directions http://www.science.uva.nl/research/scm/optcorn/ripple/ripple.html . Weird!
Fig 1. A micrograph of the barbules in a peacock feather.*
Fig 2. A sketch of the structure of the barbule. The regular structure reflects certain colors.*
*H. Durrer, 'Schillerfarben beim Pfau' (Pavo cristatus L.). Eine elektronenmikroskopische Untersuchung, Verhandlungen der Naturforschenden Gesellschaft Basel 1962, 73, 204-224.