Mountain Bluebird, Rimrock Springs, Madras, Oregon, 14 June 2008 by Greg Gillson.
When is a blue bird not blue?
The hint to this riddle is in the sky. What color is the sky? Why does the sky appear blue in the day and black at night?
Just as there is no blue pigment floating around in the sky, there is no blue pigment in a bird's feathers. The gasses and dust in the atmosphere affect the sunlight--absorbing red and scattering blue--making it appear that they sky is blue. Likewise, a layer of cells within the feather barbs reflects back the blue color frequency to our eyes.
The primary pigments (biochromes) that color bird feathers are melanins and carotenoids. Melanins produce dark colors--black, browns, and rust. Carotenoids produce red, orange, yellow. There are some birds (turacos of Africa, and bustards of the Old World) that have additional pigments that create those species' green, pink, and red plumage colors.
What about green birds? For the most part, the green color we see from a bird's feather is blue light reflected back at us through yellow-pigmented feather cells.
Iridescence produces the brilliant metallic reflections that turn a hummingbird's black gorget feathers a startling red, or catches the sunlight just right to make a drake Mallard's head a brilliant metallic green or purple.
Like a rainbow produced by light striking a prism, sunlight reflects off the structure of barbules (smaller feather structures that act as a zipper to hold the barbs together making the feather vanes).
Thus, the blue iridescent highlights on the head of a Brewer's Blackbird is produced slightly differently than the blue on the Mountain Bluebird above. But in neither case are there any blue pigments involved.
Information for this article is from the entry: "Colors of feathers," The Audubon Society Encyclopedia of North American Birds, 1980, by John K. Terres.