Blue Light as a Feather

Ornithologists will be revising a century-long misconception in textbooks with a report upsetting the prevailing view about why some bird feathers appear blue. The work, published in this week's issue of Nature, raises questions about how feather growth is controlled genetically to create specific colors, which in turn dictates how birds choose their mates.

Led by ornithologist Richard Prum at the University of Kansas, Lawrence, a group of mathematicians and ecologists set out to explain a growing number of anomalies raised by Rayleigh's theory. Developed in the 19th century, the theory holds that feather barbs--the branches coming out of a feather's spine--are blue for the same reason the sky is blue: because molecules in both scatter light at the blue end of the spectrum. To test this, Prum's team gathered feather barbs from a blue South American bird, the cotinga (of which only males are that color), and magnified them up to 30,000 times with an electron microscope.

Here the mathematicians stepped in, analyzing the distribution of air bubbles in the barbs. The air bubbles, they found, were packed so tightly that the light waves scattering off each bubble interfere with one another--findings that contradicted Rayleigh's theory. Prum concluded that the blue color of cotinga feathers occurs because of light waves interfering with one another--not because the bubbles are scattering light independently, each producing the color blue. Now, says Prum, ornithologists can study female preference "all the way down to the nanometer scale."

Although cautious about how broadly--and to how many species--Prum's work applies, experts are enthusiastic and curious about the evolution of the blue hue in cotinga necessary for finding a mate. How this is controlled genetically to keep the air bubbles at a precise distance is a mystery, says Edward Burtt, a visiting research professor in biology at the University of Utah, Salt Lake City.