https://www.wsj.com/articles/the-very-different-ways-cardinals-and-blue-jays-get-their-hues-1511284822
The Very Different Ways Cardinals and Blue Jays Get Their Hues
What makes cardinals red and blue jays blue? Helen Czerski on the hidden science of nature’s ways of making color
While doing oceanographic research in Rhode Island a few years back, I spent hours watching the endless feathered soap opera on my garden bird feeder. My favorite in the cast was the male northern cardinal because of its stunningly vivid red color, topped off with a jaunty crest. The large and noisy villain, a blue jay, would tip every other bird off the feeder and squawk at any rival who didn’t take the hint.
The jay’s bright blue plumage made it easy to forgive. I was enchanted by the color, because where I grew up in northwest England, all birds seemed to be either brown or brownish-beige. As a scientist, it was also striking to me that these two birds were flaunting two totally different ways of making color.
A child with a paintbox will simply dip a paintbrush into the right color and transfer it onto something else. The male northern cardinal does something similar. The important ingredient is a pigment, a molecule that absorbs some colors and reflects others. A red pigment molecule will absorb the blues, greens and yellows so that only the red is reflected back to your eyes. The absorption happens because of the structure of the molecule.
The cardinals use pigments called carotenoids, which are common in many leaves and seeds and tend to be reds, oranges and yellows. I find it fascinating that the birds can’t make these pigments—no animal can. But plenty of plants make them, and so animals get the pigments from their diet. A cardinal is a living splash of paint—the bird has just transferred the right pigment into its feathers, and now it’s red.
The cardinals use pigments called carotenoids, which are common in many leaves and seeds and tend to be reds, oranges and yellows. I find it fascinating that the birds can’t make these pigments—no animal can. But plenty of plants make them, and so animals get the pigments from their diet. A cardinal is a living splash of paint—the bird has just transferred the right pigment into its feathers, and now it’s red.
But pigments never turn a bird blue. The color of the blue jay comes from the architecture of the protein (keratin) that its feathers are made from. The outer layer of keratin is full of tiny air pockets, and as light waves flood in, they bounce off the boundaries between air and protein.
The critical part of this architecture is the spacing of the pockets in the protein. They are structured so that a red light wave, which is relatively long, can bounce off, overlap with another red wave that bounced off another pocket and, through this encounter, get canceled out. The same happens for yellows and greens.
But blue light has a much shorter wavelength, and the pocket spacing means that when a blue wave meets another scattered blue wave, they line up, reinforcing each other. Blue light is the only color that escapes intact, and so this spongelike protein structure is how the blue jay earns its name.
But blue light has a much shorter wavelength, and the pocket spacing means that when a blue wave meets another scattered blue wave, they line up, reinforcing each other. Blue light is the only color that escapes intact, and so this spongelike protein structure is how the blue jay earns its name.
This is called structural color, and it exists because of the way a nanoscale texture like these keratin pockets interferes with light waves. We’re most familiar with this phenomenon as iridescence, when light reflects off many thin layers to produce shimmering colors, depending on the direction from which you look, like a peacock feather or the surface of a CD.
Green parrots, snakes and frogs use both the pigment and structural methods at the same time. Their green is a mixture of blue structural color and a yellow pigment, which is why sometimes you’ll see a dead snake that looks blue. The chemical pigment has broken down as part of the decomposition process, but the nano-size architecture is untouched, leaving only blue.The trick of a blue feather is that while the air pockets don’t have a perfectly regular arrangement—they’re slightly jumbled—the average spacing is the same in all directions. So the feather looks blue from every angle.
