Thursday, December 24, 2009

Bird Coloration

Nature Watch
By Susan Benson,
CNHM Director of Education

A blue jay darts across the yard. An American goldfinch shows its more camouflaged winter colors that are so drastically different than the summer yellow and black. Even the black and whit e of a chickadee against the snow is a striking color contrast. Color is not always what it seems in the bird world, in spite of the amazing variety of colors we see. Still, it seems to bring a gasp of pleasure when we see these colored wonders.

For most birds, color comes from pigment, which are chemical compounds located in the feathers or skin. Pigments absorb part of the white sunlight as it hits, and then reflect just part of the color spectrum, which is what the observer’s eye sees. For example, a cardinal has pigment that absorbs all the wavelengths of light except the ones that, when they enter our retina receptors, appear red. When we see black color, no light is reflected. When we see white color, all wavelengths are reflected. Melanin pigments produce blacks, browns, grays, and beige colored feathers. The pigments that produce yellows, reds, and oranges are called carotenoid pigments. These carotenoids are not produced within the bird’s body, however. Birds of these colors rely on their food – pink flamingos depend on carotenoid-rich crustaceans for their color, and the yellow goldfinches get their pigmentation from seeds they eat.
The blue of a jay that we see is not coloration based on pigments. In fact, blue and iridescent colors are "structural colors," produced in special cells in the feather barbs. For us to see “blue,” there are microscopic box-shaped cells that scatter the light, favoring the shorter blue wavelengths. Iridescent colors are produced by from modified barbules that cause wavelength changes based on the angle of reflection. These special cells split the sunlight, making iridescent colors dependent on the angle of the light. A ruby-throated hummingbird aligned just right makes a shimmering sight, but when turned wrong, the color goes black.
Many of our camouflaged birds exhibit what is known as "countershading.” These birds have dark backs with gradual lighter shading until the belly is white. Countershading tends to eliminate an abrupt shadow, absorbing bright light from above while reflecting light below where light is dim. “Disruptive coloration” is the use of patterns that break up the outline of the bird to avoid detection. Killdeers are an excellent example of this disruption, allowing the bird to blend in with the color of its background.
Why is coloration important? The earliest theory made by Darwin regarding bird coloration was that the bright coloration of males evolved through female choice of the most attractive male plumage. Another “color” theory is that bright colors can intimidate predators. Or, if we consider the bright coloration some species, such as monarch butterflies or coral snakes use to warn predators, it could be that the more colorful the bird the more unpalatable. It is believed that birds identify themselves to other flock members through color patterns visible in flight. Bright colors might help to deflect predator’s attention away from nesting sites. Colors inside of mouths of open chicks may stimulate parental feeding and help guide them to where “x marks the spot.” These are all interesting examples of how birds use their coloration.
Perhaps bird songs did not evolve for our human enjoyment, nor did bird colors evolve to delight our eyes. In spite of this, we benefit greatly from the joy they bring. Millions of dollars are spent every year as humans attempt to draw them closer to their homes. Birds are a part of many people’s regular conversation. We even bring their sounds and bright colors into our homes as pets. Whatever their color, they are an integral part of our human lives.
For 40 years, the Museum has served as a guide and mentor to generations of visitors and residents interested in learning to better appreciate and care for the extraordinary natural resources of the region. The Museum invites you to visit its facility and exhibits, the Curiosity Center and Brain Teasers 2, in Cable at 13470 County Highway M. Also find us on the web at to learn more about exhibits and programs.

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