Blue jays are not blue.

At least, they’re not blue in the way that a cardinal is red. There are two types of color in birds, pigmented and structural. The cardinal is red because its feathers really are red. They are loaded with pigments called carotenoids. These pigments are also responsible for the gold in goldfinches and the yellow in yellow warblers. They are the reason carrots are orange.

Many dark colors are produced by pigments called melanins. Depending on the concentration and mix of these pigments, a bird’s color can range from black to brown to dull orange. Melanin is also helpful to birds because it adds rigidity to feathers. Melanin in the black wingtips of gulls makes the flight feathers more resilient.

Animals produce their own melanins. Even humans produce melanins when tanning, which darkens the skin. But the building blocks for carotenoids come from diet. Flamingos get their pink from the color of the shrimp they eat. Deprived of shrimp, the flamingo is white. There are blue pigments in nature. The blue in blueberries is a pigment. Chlorophyll is a green pigment. Yet birds cannot convert these pigments to color their feathers.

Blue and green are oddball colors in the animal kingdom. They are structural colors. In birds, they are caused by microscopic structures in the feather that refract light like a prism. When this light passes through a dark layer of melanin, some wavelengths are filtered out, and the bird looks blue. In Maine, blue jays, indigo buntings and bluebirds all get their color from structure, not pigment. If the light passes through a yellowish pigment, the bird looks green. Most parrots are green because of structural color.

Structural color exists in insects and amphibians, too. The blue morpho is a large tropical butterfly that glows a brilliant blue.

Iridescence is a structural color. Refracted light is why a hummingbird is ruby-throated. It’s why grackles and starlings turn shimmering purple in the sun. It’s why the glossy ibis is glossy. Iridescence is apparent only when the light hits the bird at certain angles. Otherwise, all you see is the dark of the melanin.

Test it yourself. The next time you find a blue jay feather, hold it up against a bulb so that the light passes through it. Or crush the feather, destroying the intricate structures. In both cases, the blue disappears.

There are more than 9,000 species of birds in the world, and the color variation is astonishing. That’s a clue that vision is the most important of senses among birds. Of all terrestrial animals, birds have the largest eyes relative to their body size. Humans have two types of color receptors in their eyes. Birds have four, enabling them to see a wider range of colors, including color in the ultraviolet spectrum. To us, male and female chickadees look alike. But in the ultraviolet range, they look different, and I’m guessing that’s a turn-on for the males.

Furthermore, some birds rely on ultraviolet light to detect a meal. The waxy skin of fruits in the jungle reflects in the ultraviolet spectrum, and a parrot would see it more brightly than we would, finding it more easily. Some rodents leave scent trails and urine that reflect in ultraviolet light. It is possible for some raptors to track a vole, even at a considerable distance, by following its reflected trail.

Everyone knows that raptors have sharp vision, but many birds can also resolve rapid movements faster than we do. It’s what enables a sharp-shinned hawk to dash through a forest, nimbly avoiding limbs and branches. For us, it would all be a blur. There are, of course, drawbacks to having eyeballs so laden with receptors. Hawks have poor night vision and have no choice but to roost in darkness.

On the other hand, owls have many more rods than cones in their eyes, allowing them to see in the dark, but sacrificing some ability to see color. Even then, owls find a way to compensate. Besides melanins and carotenoids, there is a third set of color-producing pigments called porphyrins. These are often found in the plumage of owls and they can see these colors well enough to give each other social clues.

Birds use color to attract mates, repel rivals, signal anger, determine pecking order and aid concealment. Birds get brighter in spring when breeding is important and duller in autumn when camouflage is important. To achieve the same results, humans shop at the mall.

Bob Duchesne serves as a Maine Audubon trustee and vice president of its Penobscot Valley Chapter. Bob developed the Maine Birding Trail, with information at Bob can be reached at