One winter day on a ride in the country, my mother and I saw a pair of Eastern Bluebirds on a mesquite tree near a pond. We were a long way off but we had binoculars, and the birds were bathed in sunlight so we got a wonderful look. The male was brighter than the female; both had rusty throats, clear white bellies, and heads and wings and backs of cerulean blue.
     The birds were in no hurry, and would change position from time to time, giving us excellent views of back, sides, and front. The light on those blue feathers seemed to shimmer — the sight as a whole felt to us like some great gift. We passed the binoculars back and forth, and marveled at our luck.
     Henry David Thoreau once said that the Bluebird carries the sky on its back, and to this, my mother and I would gladly testify. The blue of those birds seemed a distillation of the winter sky behind them.
     In point of fact, the connection between the sky and the blue in a Bluebird is more than just poetic — there is actual science involved! And the pleasure my mother and I felt that day was due, in part, to the fact that those particular Bluebirds were drenched in sunlight.
     Without light, the blue in a Bluebird is not blue at all, but turns to grey. A Bluebird in the shadows is most difficult to see, while the red of a Cardinal or the yellow in a Warbler remains bright no matter where it is.
     This is because there is no such thing as blue pigment. Well, almost none — a few exotic birds are said to possess blue or green pigment, but in all ordinary blue birds — such as Bluebirds, Blue Jays, Blue Grosbeaks, and Indigo Buntings — blue pigment is nowhere to be found.
     So how do our eyes deceive us? There are two kinds of feather color: pigmented and structural color. Pigmented feathers contain molecules that reflect specific wavelengths to produce red, brown, yellow, and black — which means that red feathers are really truly red.
     Certain birds must ingest certain foods to retain their pigment. Pink Flamingos would not be pink for long without a steady diet of pink brine shrimp.
     Structural color results from a trick of the light: blue, green, and iridescent birds derive their color from the structure of their feathers, where tiny ridges and air sacs refract and scatter the light and create the impression of color. The distance between these surface irregularities determines which colors we see.
     Blue sky results from a similar refraction of atmospheric light. Just as the void of sky at night is black when the light is gone, so the blue in a Bluebird fades in the shadows.
     Structural color changes with the angle of view. A Bluebird can seem turquoise from one angle, more deeply blue from another. 
     Wet a blue feather, and it turns brownish grey, but once the surface dries the refraction is repaired and the blue seems to reappear.
     I didn’t know any of this when I tried to photograph feathers, but I do remember having quite a time getting the blue ones to show up blue.
     All this talk about color and the illusion of blue calls to mind the Bluebird of happiness. Perhaps happiness, like the blue in a Bluebird, is not only elusive but largely imaginary. Still, it’s always well worth a glimpse.

(Heartfelt thanks to the folks of texbirds.org for explaining Bluebird blue.)