The Not-So-Blue Jay
Just to set the story straight, blue jays (Cyanocitta cristata) are not blue. They're brown. Sort of.
To be more precise, blue jays, and other blue birds, don't have any blue pigment in their feathers. Instead, their feathers are composed of tiny, specialized cells, generally no larger than 0.6 microns. While a feather is growing, the keratin in the cells elongates and separates from water. When the cell dies, the water evaporates, leaving the keratin in a honeycombed structure, called “barbs” in blue jays. These cells absorb red and yellow light and reflect blue light, which is what we see. The size and shape of the honeycomb creates different shades of blue.
Blues, violets and greens are all produced this way, and can be enhanced by the darker layers underneath. An iridescent shimmer is produced when larger, less homogenous spaces are found on the same feather. All of these colors are called structural colors, clearly because they aren't produced by pigments but by the structure of the feathers. The man who discovered this type of light reflecting structure was a British physicist by the name of John Tyndall. The color of the sky, “Tyndal blue” is named for him, as the color of the sky is produced by the reflecting and scattering of light off small particles.
But the blue scattering doesn't end with the sky. Smoke coming from motorcycles and other two-stroke engines often appears blue from reflected light. Even blue eyes are a result of the Tyndall effect. All eyes have a turbid layer in the iris. Brown eyes have melanin in that layer, but blue eyes allow light in. Part of it is then reflected and scattered, leaving baby blues.
Photo Courtesy of Michael Baglole:https://www.flickr.com/photos/mbaglole/