In thinking about how colors are created in bird feathers, I wondered if it was similar to how color is formed in butterfly wings. I learned that yes, it is very similar, and that bird feather color has evolved in several ways, from pigmentation present or as a result of light refracting through the layered structure of the feather.
Pigments are colored material found in plants, animals, and nearly every physical substance in nature. Pigmentation in birds comes from three different sources: melanins, carotenoids, and porphyrines.
Melanins are tiny bits of color in the feathers of birds and in their skin. Melanins produce colors from palest yellow to rusty red browns to the richest black, depending on where the melanin is located and in what degree of concentration. Feathers with melanin are the strongest of all. A bird’s flight feathers are the most susceptible to wear and usually have the highest degree of melanin.
Carotenoids are produced by plants. Birds that eat specific plants, or eat something that has eaten the plant, acquire pigment from carotenoids. A carotenoid-rich diet is responsible for the beautiful vermillion feathers of the Northern Cardinal, as well as the electrifying cadmium yellow of the male American Goldfinch. Another example is the pink feathers of the flamingo, which also have a diet rich in carotenoids that come from the crustaceans that they eat, which ate algae. Melanins and carotenoids can interact to produce feathers such as olive green.
The third group of pigments are called porphyrins and they are the rarest, found only in a handful of bird families. Porphyrins are produced by modified amino acids and all share a common trait, which is to fluoresce bright red when exposed to ultraviolet light. Porphyrins are found in some pigeons, owls, and turacos.
The intensity of the red of the Northern Cardinal is an example of how feather color plays an important role in the survival of a species. Cardinal foods high in carotenoids include rose hips and dogwood berries. The brightest red birds usually have superior breeding territories, with the greatest abundance of their preferred foods. The reddest birds make the most successful parents because of their ability to bring an increased amount of food to the nestlings. When Cardinals are raised in captivity on a diet lacking in carotenoids, with each successive molt, the feathers become paler and paler.
Like butterflies, birds can see color in the ultraviolet spectrum (we humans cannot). Perhaps the way we see birds is entirely different from they way they see themselves!
Part Two Structural Color continued tomorrow.