Spring equals color. Wildflowers bloom. Butterflies appear. And, of course, neotropical migratory birds return.
Though migration is well underway, the brightest and most colorful birds have not yet arrived. Give them a few more weeks. But get your hummingbird feeders up today; they’re coming back early this year (www.hummingbirds.net/map.html).
To most of us, nature’s colors are eye candy, an aesthetic delight that defines a season. But it means so much more to birds.
Obviously, birds see colors (why else would they be so colorful?), but the structure of their eyes suggest their color vision is even better than ours.
Many, in fact, can see ultraviolet light. To appreciate the importance of color that science has assigned to birds, just peruse a field guide to North American birds: scarlet tanager, painted bunted, rose-throated becard, black-throated blue warbler.
Names celebrate colors
Many common names celebrate nature’s palette of colors. And many tropical species are even more colorfully named: emerald toucanette, flame-colored tanager, rainbow-bearded thornbill and sparkling violetear, to name just a few.
Coloration in birds is one of nature’s great compromises. On one hand, they want to be inconspicuous to predators. On the other, they want to be noticed for social reasons such as courtship and territoriality.
Clearly, a whip-poor-will and an American woodcock disappear into the leaf litter of the forest floor, and yet a Baltimore oriole and a male northern cardinal stick out like sore thumbs in the tree tops. But that is to human eyes.
Many predators don’t see the world the way we do, so what’s conspicuous to us is not necessarily so to them. Most mammalian predators and owls, for example, do not see colors at night, so brightly colored feathers are not a problem.
These predators see colors as shades of gray. Hawks and other diurnal avian predators see colors well. So what’s cryptic to one predator can be obvious to another. But that doesn’t account for context.
A male goldfinch or scarlet tanager may seem impossible to miss, but in the sun dappled greenery of the tree tops even brightly colored birds can be difficult to distinguish.
I can’t count the number of times I’ve been fooled by a flash of sunlight reflecting off a leaf twisting in the breeze.
Seeing a brightly colored bird well enough to catch it in brightly lit foliage is no easy task for any predator. And on dimly lit shady days, bright colors lose their luster.
Plus many birds can behaviorally adjust the conspicuousness of their colors. A red-winged blackbird can cover his crimson epaulets by fluffing other feathers.
If a meadowlark wants to be seen, it simply faces the target to expose its golden chest. To vanish, it just turns around and its cryptically colored back disappears into the vegetation.
Well-camouflaged birds such as woodcock, shorebirds and grassland sparrows eschew bright colors in favor of visual and vocal displays.
Elaborate courtship displays and dances and easily heard songs are as useful as bright colors to attract attention.
The colors that birds exhibit, whether bright or drab, are caused by the wavelengths of light reflected from feathers to the viewer’s eyes.
A male cardinal is red, for example, because its feathers reflect red light and absorb other wavelengths.
Light processing systems
The colors that we see in birds result from one of two light processing systems — pigmentation and feather structure.
Pigments are chemicals found in the feathers and skin. They may be inherent in the bird or derived from foods the birds eat.
Conversely, the microscopic physical structure of feathers reflects specific wavelengths of light to produce specific colors. If the structure of the feather is destroyed, the color disappears.
Pigments are responsible for most of the colors we see in birds and there are three main types: melanins, carotenoids and porphyrins.
Melanins are the most important and, depending on their concentration, can produce colors ranging from black to pale yellow.
Melanins are made from amino acids which are extracted from proteins in the diet.
Over the next few weeks, I’ll continue to examine the natural history of color in birds.