A flash of red signals the return of a male rose-breasted grosbeak. A lemon drop bouncing across a country road means yellow warblers are back. And a patch of deep blue atop a withered snag tells me indigo buntings have arrived.
Spring migration and its parade of color are under way. The colors of birds result from chemical pigments or feather structure.
Melanins
Melanins occur as microscopic granules in skin and feathers. Depending on their concentration, they can make crows and ravens black or red phrase screech owls rusty.
Birds manufacture melanins from amino acids. Melanins also give feathers strength and resistance to the elements.
That’s why many white or lightly colored birds have black wing tips. Think gulls, terns, gannets, shorebirds and pelicans.
White feathers, those that lack pigments, wear out faster than pigmented feathers.
Carotenoids
Carotenoids are the pigments that make cardinals red, orioles orange, and warblers and goldfinches yellow. But unlike melanins, which birds synthesize, carotenoids occur only in plants.
To acquire carotenoids, birds must eat these plants or something else that has eaten these plants.
Porphyrins
A third pigment, porphyrins, is also manufactured from amino acids. They interact with melanins to form browns, reds and greens and play a role in coloring eggshells.
When genetic mutations occur that interfere with the production of pigments, we see strange results.
The complete absence of pigments is rare and results in true albinos. Partial albinos, those that have a reduced amount of melanin, are much more common and these individuals appear washed out.
Readers often send me photos of pale house finches and robins. This condition is called leucism and these individuals are called leucistic.
Sometimes the absence of melanins is limited to particular parts of the body. I once had a white-sided, white-winged, red-winged blackbird at my feeders. I’ve seen crows with white wing patches. And a robin that lacks melanin can appear white, but still have a rusty breast.
Structural color
Though pigments are responsible for most of the colors we see on birds, some colors are structural. They result from the way certain wavelengths of light are reflected from microscopic feather structures. Iridescent colors are formed this way.
The red on the throat of male ruby-throated hummingbirds, for example, is a structural color. Watch a hummer at a feeder and notice how the red disappears as the viewing angle changes.
Colors formed by pigments do not change if the angle of viewing changes. But not all structural colors are iridescent.
Tiny pockets of air within the cells of feather barbs sometimes scatter shorter wavelengths of incoming light. This is typically how shades of blue are formed.
Blue pigments are rare
In fact, blue pigments are rare in birds, occurring only in some tropical species. The blues we see in blue jays, bluebirds, indigo bunting and blue grosbeaks are structural.
A layer of melanin just beneath the tiny light scattering pockets of air absorbs the longer wavelengths of light. This creates a dark background so the blues we see are vivid and intense.
To see a blue that’s bluer than blue, a blue that’s enhanced by a melanistic background, take a good look at a male indigo bunting in bright shade.
In the lab, the difference between pigments and structural colors can be demonstrated by grinding up blue feathers into feather dust. The dust is dark, not blue.
Grind up chemically pigmented cardinal feathers, on the other hand, and you get red feather dust.
Many purposes
The variety of feather colors that birds exhibit serves many purposes. Cryptic colors offer protection. Owls, whip-poor-wills and woodcock are difficult to spot when perched in vegetation.
Disruptive colors, like the bars on a killdeer’s chest, break up its outline on gravel beaches. Sub-adult and adult plumages signal sexual maturity. And colorful markings enable birds to recognize each other by species and sex.
For example, male flickers have a black malar stripe (mustache); females do not. In one classic field experiment, a researcher painted a malar stripe on female flickers and their mates chased them away.
Perhaps that’s why we shouldn’t judge books by their covers.
