Nov. 5, 1998 LAWRENCE -- Essayist Henry David Thoreau said that the bluebird carries the sky on its back.
For more than a century, scientists have agreed, saying that blue feathers look blue for the same reason that the sky does.
Now, a University of Kansas scientist and his colleagues say that's not so. They have found that feathers look blue for the same reason that oil slicks do.
Rick Prum, curator of ornithology at the KU Natural History Museum, said, "These results apply to many common garden birds, including blue jays, indigo buntings -- and, yes, Thoreau's bluebird."
He and his colleagues published their discoveries about the blueness of bird feathers in the November 5 issue of the journal Nature.
Blue feathers and blue sky are both the product of the interplay of light with the structure of materials found in the two, Prum said. That is, neither is blue because of pigment, as jeans are.
But that's where the similarity between the blues ends, Prum said.
The sky is blue because of molecules of gas and other particles that scatter light waves at the blue end of the color spectrum, Prum said.
The blue that you see in an oil slick, or in a blue bird feather, doesn't come from this kind of scattering, Prum said.
"If you open any ornithology text, you'll see the statement that bluebird feathers are blue because of this scattering, and they're wrong," Prum said. "That dogma is a century old."
The blue in oil slicks and feathers results from differences in the distances traveled by light waves that are reflected off of each.
Light waves traveling toward your eye from an oil slick are reflected off both the top and bottom of the slick.
Because of the different distances the waves have to travel, the peaks and troughs of most of them won't line up. So most wavelengths of light cancel out and the colors associated with those wavelengths aren't visible.
The troughs and peaks of a small range of colors do line up, though, and are brilliantly reinforced, Prum said. With an oil slick, the reinforced color is blue.
The same general principle is at work with feathers, he said.
In his experiments, Prum cross-sectioned feather barbs taken from a blue South American bird called the cotinga. The barbs are the branches that come off a feather's central spine.
When the cotinga's barbs are magnified 20,000 to 30,000 times, Prum said, you can see they are made of a spongy 50-50 mix of keratin and air bubbles.
Keratin is also a major constituent of human fingernails and hair.
In their research, Prum and his colleagues analyzed the distribution of the air bubbles. They found that the bubbles in the feather barbs are too close to each other for the feathers' blueness to be a product of the scattering of blue light waves, as in the case of the sky.
But they also found that the bubbles were distributed regularly enough that they could produce small but predictable differences in the distance that light waves that struck the bubbles traveled to the eye from the barb.
As with the oil slick, this difference in distance traveled was responsible for the blueness.
"The amazing thing is that as the feathers grow, the size of these tiny air cavities in the feathers has to be exactly right to create the right color," Prum said. "If the air bubbles are different in size by, say, 5 millionths of an inch, the result is an entirely different hue."
Prum said that animal communication through color "is a fascinating field. In order to understand the meaning of color signals -- if they have a meaning -- and how they came to evolve, we have to understand how those colors are produced."
Besides Prum, others working on the project were Rodolfo Torres, KU associate professor of mathematics; Scott Williamson, then a KU undergraduate and now a KU graduate student in biology; and Jan Dyck, of the University of Copenhagen.
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