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Robert Miller: Birds of brilliant feathers

Updated 12:06 am, Saturday, June 29, 2013
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There was a male mourning dove at the bird feeder in my yard a week or so ago. It was early morning, the sun was out after a night of rain, and the world was shining.

And I saw something I'd never seen before, or never seen so well -- a beautiful patch of pale pink iridescence on the dove's neck.

As the dove moved, that shimmering patch changed in shape. Mourning doves are beautiful anyway. This one was just that much more so.

There also was a grackle at the feeder that morning, black and glorious and shining as well. There was a red-bellied woodpecker whose bright red head was glowing.

But the red feathers are different than the iridescent shining on the dove's throat or the grackle's black sleek feathers.

The red feathers are pigmented -- the feathers are colored. If you plucked one off the woodpecker's head it would be red.

The colors in iridescent feathers however, are a matter of structure -- they catch and refract light. When we see an indigo bunting or a blue jay, it's that refracted light our eyes see as blue. One of those feathers, on the ground, seen at a different angle, could look drab and brown.

Richard Prum defends those feathers, however. To him, they're very blue.

"If a bluebird isn't really blue, well then, a cardinal isn't really red,'' he said.

Prum is the William Robertson Coe professor of ornithology at the Peabody Museum of Natural History at Yale University. He is one of the world's great experts on the biology and physics of bird feathers.

For him, seeing colors is a matter of physics -- of how light, surfaces and our eyes interact. If a feather is iridescent and appears to be a brilliant blue or green to our eyes, Prum reasons, it is, essentially, that color.

The colors we see in bird feathers are either the result of the pigments in the feathers, or their structure.

The pigmented colors, Prum said, are the reds, yellows, blacks and browns. Those yellower-than-a-lemon goldfinches we see have yellow feathers.

Structural colors are the blues, greens, and the purple-black sheen of grackles. The only exception to this is the reds on hummingbirds. Those ruby throats we see can disappear with a change of light.

Prum said that colors are important to birds for mating, but also for social grouping -- bluebirds see that they're different than indigo buntings. Birds of a feather really do flock together.

"For a blue jay to be a blue jay, it has to be blue,'' Prum said. "If it were violet, it wouldn't do too well.''

But feathers, even pigmented ones, can shine in the sun as well. Crows can look that way. So can the red heads of woodpeckers. They are just presenting the sun with enough of a shiny surface to reflect light. They're glossy.

Prum said, that birds, in discerning color that tell them so much about their world, see more color than we do. They pick up on cues we cannot even imagine.

"We have three color receptors -- red, green and blue,'' Prum said. "Birds have a fourth color receptor in the ultra-violet range.''

So while we can glory at the beauty of a pink splotch of iridescence on a mourning dove's neck, another mourning dove is seeing that and more.

"Compared to birds, we're relatively color-blind,'' Prum said.

bmiller@newstimes.com; 203-731-3345