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Blue over bluebirds

Posted: March 1, 2013 3:53 p.m.
Updated: March 4, 2013 5:00 a.m.

Last night, my daughter wanted to draw a bird. Elated she was showing common interest, I quickly answered her request to view the “bird book” in order to find the perfect candidate. She thumbed through the pages, impressed by the vast array of coloration found in our native species. She finally landed on Eastern Bluebird, pointed to the most colorful photo and asked, “Is that the mommy or the daddy?” I told her it was the daddy, and that in nearly all cases, the daddy bird is the more colorful of the two. Dumbfounded, she perused other pages, asking me in each instance the same question as before, further substantiating her daddy’s words. Then came the silence. Then came the pouting. Then came these words, “I don’t want the daddies to be prettier than the mommies.” And in an instant, she put the bird book down and went back to her princess dolls, all of which obey her preconceived precepts of beauty.

One day I hope to explain to her that the colors of feathers, for many reasons, are well worth celebrating. As for the difference in genders, biologists call this sexual dimorphism, and it happens in many species for many reasons. In birds, the female is often the primary incubator of eggs. Her drab colors are better considered camouflage, not willful ugliness as my daughter assumed. In contrast, the male has bright coloration as a means to attract the female. While we take it for granted, bright coloration is an uncommon occurrence in nature, found mostly in poisonous animals and birds. This makes sense. Bright colors are obvious to predators, and one must have a ready defense if they choose to don such a display. Poison works well for obvious reasons. For birds, flight provides a means of escape. For this reason, flight is thought by some to be the very phenomenon allowing for the development of bright coloration in birds. Otherwise, it would be hard to get away with such splendor.

Feather color is the result of either structure or pigment. The bright, iridescent colors we associate with our Ruby-throated Hummingbirds are structural. Tiny air pockets inside the feathers bend light, separating it like a prism and causing you to see different colors on the throat depending on the angle between you, the bird, and the sun. In teaching my natural history students to identify birds by color, this is often a frustration, since such colors are not constant. Other local examples of this are the backs of Common Grackles. The blue on my daughter’s bluebird works similarly, the only difference being that all wavelengths except blue are cancelled out by the feather structure. This all seems magical, but if you need proof of it, try this experiment the next time you find a blue feather. Hold it between your eye and the sun, and watch the blue disappear. The change occurs because structural colors depend on refraction and reflection of light. By holding the feather between your eye and the sun, you eliminate this effect. The only color you see is a dark gray, which represents another means of color production, that of pigmentation.

Pigment accounts for colors such as red, yellow, brown, and black. Red and yellow are obtained from carotenoid pigments found in the diet of birds. Our Northern Cardinals and various warblers exhibit this phenomenon. Browns and blacks are the result of the pigment melanin. Melanin is associated with compounds that strengthen the structure of the wing. For this reason, areas vulnerable to wear and tear, such as wingtips, are often dark in color. Look for this common trend in birds like hawks and seagulls.

Feathers are full of amazing features, and a column on coloration only strikes at the surface. We can investigate feathers more fully in the future. In the mean time, try not to lament over the feather differences donned by our birds. Despite my daughter’s disappointment, the differences are worthy of adoration, especially when you consider their adaptive significance and multiple means of manifesting color.


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