Mirror, mirror on the wall, who's the fairest of them all?
As one might expect from the amazing diversity of colors and patterns exhibited by more than 9,000 bird species found in the world, birds can see color. In fact, they can discriminate a greater variety of colors than humans; as some birds can see into the ultraviolet range.
The colors in the feathers of a bird are formed in two different ways, from either pigments or from light refraction caused by the structure of the feather.
In some cases feather colors are the result of a combination of pigment and structural colors. The greens of some parrots are the result of yellow pigments overlaying the blue-reflecting characteristic of the feathers.
Pigments are colored substances that can be found in both plants and animals. The coloration created by pigments is independent of the structure of the feather. Pigment colorization in birds comes from three different groups: melanins, carotenoids, and porphyrines.
Melanins occur as tiny granules of color in both the skin and feathers of birds. Depending on their concentration and location, melanins can produce colors ranging from the darkest black to reddish browns and pale yellows.
Melanin provides more than just coloration. Feathers that contain melanin are stronger and more resistant to wear than feathers without melanin. Feathers without any pigmentation are the weakest of all. Many otherwise all white birds have black feathers on their wings or black wingtips. These flight feathers are the ones most subject to wear and tear. The melanin causing the tips to appear black also provides extra strength.
The red of the Northern Cardinal comes from a class of pigments called carotenoids. Carotenoids are produced by plants, and are acquired by eating plants or by eating something that has eaten a plant. Carotenoids are responsible for the bright yellows seen in goldfinches and Yellow Warblers as well as the brilliant orangish yellow of the male Blackburnian Warbler. Carotenoids can interact with melanins to produce colors like the olive-green of the female Scarlet Tanager.
Porphyrins, the third pigment group, are produced by modifying amino acids. Although the exact chemical structure of each porphyrin differs, they all share a common trait. They fluoresce a bright red when exposed to ultraviolet light, much the way certain rocks and minerals are known to do. Porphyrins produce a range of colors, including pink, browns, reds, and greens. Porphyrins are found in some owls, pigeons and gallinaceous species. They can also produce the brilliant greens and reds of turacos.
When pigments are present (or absent) at unusual levels the appearance of a bird can change dramatically. These color abnormalities, while not common, do occur on a regular basis.
Adding to the diversity of avian colors are colors produced by the structure of the feather. The best known example is the gorget (throat feathers) of many hummingbird species. The iridescent colors of the gorget are the result of the refraction of incident light caused by the microscopic structure of the feathers. The refraction works like a prism, splitting the light into rich, component colors. At certain angles little or no light is reflected back to the viewer and the gorget can appear black. As the viewing angle changes, the refracted light becomes visible in a glowing, shimmering iridescent display.
Many species of birds have feathers that exhibit iridescent colors, including the Purple Gallinule and Tricolored Heron pictured at the top of this page.
Not all structural colors are iridescent. Tiny air pockets in the barbs of feathers can scatter incoming light, resulting in a specific, non-iridescent color. Blue colors in feathers are almost always produced in this manner. Examples include the blue feathers of bluebirds, Indigo Buntings, Blue Jay's and Steller's Jays.
The blues seen in the feathers of these 3 species are structural colors. (Left to right: Indigo Bunting, Mountain Bluebird and Stellar's Jay)
If you find the feather of a Blue Jay or Steller's Jay you can see for yourself how this works. First, observe the feather in normal lighting conditions and you will see the expected blue color. Next, try back-lighting the feather. When light is transmitted through the feather it will look brown. The blues are lost because the light is no longer being reflected back and the brown shows up because of the melanin in the feathers.
The feather structures of many species also reflect light in the ultraviolet range. Some birds can see into the UV range so they may appear quite different to each other than they do to us.
In his Monograph of the Trochilidae or Family of Humming Birds, famous British artist John Gould developed a unique approach to portraying the iridescent colors seen in the gorgets of the hummingbirds he painted.
Produced in the mid-1800s, each original monograph was 16" x 22" in size, from which a limited number of prints were produced.
Making the prints was technically and artistically challenging. In an early lithographic process, Gould's original sketches were transferred to stone with special pencils or chalk. Prints were then produced by hand from the stones. Each print was then hand-colored and issued in small sets to subscribers.
Gold leaf was used to achieve the desired appearance for the iridescent areas on the hummingbirds. After the gold leaf was laid down it was overpainted with transparent oil colors, then varnished and heightened with gum Arabic. The resultant colors provided a remarkably accurate depiction of live birds.
Tropical species exhibit a wide range of colors and special adpatations. These photographs (some are very colorful) exhibit many of the special feather types and colors that have been discussed in this section.