Are Blue Jays Really Blue? How Blue Feathers Work

Why blue jay feathers look blue

Most colors in animals come from pigments, chemicals that absorb some wavelengths of light and reflect others. The black in a crow’s feathers and the brown of a sparrow come from melanin. The red of a northern cardinal comes from carotenoids, which the bird gets from its food.

The blue jay works differently. Inside the branches of each blue feather, called barbs, is a layer of keratin (the protein that also makes up our hair and nails) riddled with tiny air pockets. This spongy layer is built at a scale of a few hundred nanometers, similar to the wavelength of visible light.

When white sunlight hits this spongy keratin, the light waves bounce off countless air-keratin boundaries. Because of the spacing of these structures, blue wavelengths reinforce one another while other colors cancel out. The result is a strong blue reflection.

Underneath the spongy layer sits a core of dark melanin. This pigment soaks up the light that is not reflected, so stray red and yellow light does not wash out the blue. Without the melanin, the feather would look pale and milky rather than vivid.

Why a blue jay feather can look brown

You can test structural color yourself with a fallen blue jay feather. Look at it with light coming from behind you and it shines blue. Hold it up so light passes through it from behind and it turns a dull gray-brown, because now you are seeing the melanin, not the reflected light.

The same happens if the feather is crushed. Breaking the delicate spongy structure destroys the blue for good, leaving only the dark pigment. A feather colored by pigment, such as a cardinal’s red feather, stays red when crushed or backlit.

This also explains why a blue jay looks different in different light. In bright sun the jay glows lavender-blue, while on a gray winter day or in deep shade it can look almost slate-gray.

Other birds with structural blue

Structural color is the rule, not the exception, for blue birds. As far as scientists know, no bird makes a true blue pigment. Some well-known examples:

The common kingfisher has some of the most intense structural blue of any European bird. Depending on the angle, its back can flash from deep blue to turquoise.

The Eurasian blue tit has a blue cap that reflects strongly in the ultraviolet, a part of the spectrum people cannot see but birds can. Research on blue tits in the late 1990s found that males and females differ in this ultraviolet reflection, and that females appear to prefer males with brighter caps.

Structural blue versus iridescence

There are two main kinds of structural color in birds, and they behave differently.

Non-iridescent color, like the blue of jays, bluebirds and kingfishers, comes from the irregular spongy structure in the barbs. Because the structure looks roughly the same from every direction, the color stays much the same as you move around the bird.

Iridescent color, like the throat of a hummingbird, the neck of a pigeon or a peacock’s train, comes from very regular stacked layers of melanin in the barbules, the tiny branches off each barb. These layers act like a soap bubble or an oil film, so the color shifts dramatically, or vanishes, as the angle changes.

Structural color combined with pigment

Many bird colors are a mix of structure and pigment. Green parrots are a classic example: their feathers combine structural blue with a yellow pigment layer. The yellow filters the blue light, and the result looks green. A parrot that cannot make the yellow pigment, because of a genetic mutation, appears blue instead of green.

Blue eggs are a different story. The sky-blue eggs of the American robin get their color from a real pigment, biliverdin, deposited in the shell, as explained in why are robin eggs blue?. Blue jay eggs are not bright blue at all; they are pale bluish or light brown with darker spots.

Is blue rare in nature?

Blue is genuinely rare among animals, largely because true blue pigments are so rare. Almost every vivid blue animal, from blue morpho butterflies to blue jays, uses structure rather than chemistry. Even the blue skin on the faces of some monkeys, such as mandrills, comes from the regular arrangement of collagen fibers in the skin, not from pigment.

So the next time a blue jay lands on your feeder, you are not looking at blue color so much as at blue light, sorted by a feather built at the scale of a few hundred nanometers.