A House Wren’s gray and brown feather absorbs light and looks drab and inconspicuous. An American Goldfinch’s feather, by contrast, is designed to reflect light and be as bright as possible. Art by David Allen Sibley.

The brilliant yellow, orange, and red that you seen on many birds are not just for decoration; the colors send an important message about the health of each individual.

The colors are products of pigments known as carotenoids, which a bird gains from its diet and uses in its immune system. Consequently, a bird showing off bright yellow or red plumage is sending a message that it is healthy and eating well.

Light that passes through the yellow tip of the American Goldfinch feather reflects off the white surfaces underneath, making the bird’s plumage glow. Art by David Allen Sibley.

Every bird must balance the immediate survival value of having the pigments available for its immune system with the long-term value of attracting a mate and producing offspring. It needs to look as good as possible while using as little pigment as possible. Simple but effective adaptations allow it to do that.

A feather is a lightweight and insignificant thing, translucent and flimsy. Even a bright yellow feather, on its own, is not very remarkable. Laid on a dark gray surface, it would look drab, as some of the gray would show through the feather. Laying it on a white surface, however, would allow the yellow to shine.

How does a bird create a bright white background for the feather, duplicating this effect? Remember how feathers are arranged on a bird’s body: Normally, only the tip of each feather is visible. The feather above it overlaps all of it but the tip.

In the case of a brilliant yellow bird like a male American Goldfinch, the color is essentially the only pigment in the feather, and it is only at the tip. When the feathers are arranged normally, some light reflects directly off the yellow tip, while other light passes through it, striking the white surface of the feather below and reflecting back through the tip.

In effect, this creates a translucent yellow wrapper on a white bird, ensuring that the maximum amount of light bounces off, enhancing the color and making the bird seem to glow. It is the whiteness of an American Goldfinch’s feathers that really makes the difference.

Photo gallery: Seven pictures of vivid Scarlet Tanager.

See photos of American Goldfinch.

See photos of House Wren.

This article from David Sibley’s “ID Toolkit” column appeared in the March/April 2016 issue of BirdWatching.

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Originally Published March 11, 2016