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Blue Birds Ain’t Blue

One of the loveliest sights an ornithologist runs across is the iridescent blue found in some birds’ plumage. Sure, cardinals have red feathers and finches have yellow feathers, but if you ever run across the gleaming, almost metallic-looking blue of an indigo bunting, you won’t forget it. The colors shimmer and shine like oil on water.

You might be surprised, then, to learn that the feathers which produce such impressive displays are in themselves largely colorless.

Take some cardinal feathers, after the cardinal is done with them, and grind them up into powder. The powder will still be red. That’s because the color is the result of actual pigment in the feather. Take those shiny feathers from a bluebird or indigo bunting and grind them up, however, and their color will disappear. Why?

The glittery blues seen on bird wings are often caused not by pigment, but by the way light waves interact with the feather. Physicists call this “structural color”. Because of the layered structure of the feather, light waves entering it bounce back at different times and thus collide, or “interfere” with each other. Certain wavelengths get cancelled out while others become reinforced. The reinforced colors that come back strongly to your eye are mostly in the blue range. The result is that eye-catching display of blues.

As it turns out, there aren’t many blue pigments used by birds in general. So any time you see blue or purple on a bird’s wing, you can suspect that at least some of it is structural color. Especially if it’s got that razzle-dazzle effect.

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