Chemistry and structure combine to produce a rainbow of amazing color in feathers
By Julie Feinstein | Published: 4/21/2006
Feather colors grab our attention like few other qualities of birds. For most birds, colors are useful in teasing out their identification. Some species, of course, are highly sought after because their colors are so beautiful.
But feather color is worth understanding for its own sake. What is its nature? Where does it come from? Is it a property of the light that illuminates the birds, or does it reside in the feathers themselves?
If a cardinal’s red feather were ground into powder, the powder would be red. Grinding a green feather from the speculum of a Green-winged Teal would make yellow powder. And the feathers of a bluebird would reduce to a drab brown powder. Clearly, there is more to the color of a feather than meets the eye.
From peacock blue through swan white to oriole orange, every bird color is produced by the interaction of just two coloring systems – one structural and one chemical. Structural color results from the scattering of reflected light, while chemical color relies on a palette of pigments. Intricately arranged feather layers allow chemistry and structure to interact to produce the colors we see.
First, let’s look at what makes up a feather. Unique to birds, feathers grow in a symmetrically branching pattern, resembling the leaves of ferns. In each feather, small branches called barbs grow out of a central shaft, and smaller branches called barbules grow out of each barb. Long rows of barbules lie in flat, overlapping rows on adjacent barbs. The geometric arrangement is held in place by hook-like structures on the undersurface of the barbules that lock them together like zippers. When barbs separate, feathers look ruffled and uneven. When birds preen, they zip up the barbules and reattach the hooks, making the feathers lie smooth.
If you were to look at the cross-section of a barb under a microscope, you’d see a central core surrounded by a layer of color-producing structures and an outer region called the cortex. The simplified cross-section of a feather barb above shows the arrangement of color-producing regions.
Pigments in either the core or the cortex are responsible for some feather colors, but colors also occur in feathers in which the cortex is pigment-free. In these feathers, the layer between core and cortex – called either the cloudy zone or the spongy layer due to the appearance of a dissected feather to the naked eye – produces colors through convoluted air cavities that act as tiny light-scattering prisms. Not all feathers have a cloudy zone.
Where color begins
Parts of a feather include the calamus, which lies beneath the skin, and the rachis, the portion of the shaft that holds the two vanes. Thin barbs grow out of the rachis. This cross-section shows the areas within a barb that create color: the core, cloudy zone, and cortex.
Why cardinals are red
Pigments produce many of the colors in nature. Like paint and dye, pigments are chemicals that absorb some wavelengths of light and reflect others. The absorbed color disappears, and the reflected color is what we see. Pigments in the feathers of a cardinal, for example, reflect red light, and absorb all non-red wavelengths. Four types of pigments are found in feathers. The most common ones are melanins and carotenoids; more rarely, feathers contain porphyrins and psittacins.
Melanins produce shades of brown, black, gray, and some yellows in mammals and birds. In dark black birds, such as crows and ravens, melanin is found in both the core and cortex. When melanin occurs in the cortex only, the feather is brown or gray. Melanin within the cortex is also called foreground melanin. It makes up many of the markings in birds’ plumage patterns. The relative concentration of foreground melanin controls the intensity of the effect. In the White-breasted Nuthatch, for example, melanin occurs abundantly in the black nape and cap, less so in the gray back and wings, and not at all in the white cheeks and throat.
Carotenoids produce red, yellow, or orange feathers. Animals gain carotenoids exclusively from the plants in their diet, including flowers, roots, seeds, and fruits. Carotenoid pigments are generally fat-soluble substances like the vitamin A in carrots from which the carotenoids take their name.
The dogwood tree grows bright red autumn berries – a favorite food of the Northern Cardinal and a source of red carotenoids. When the cardinal metabolizes dogwood berries, the carotenoid pigments are sequestered in the liver and then transported to the bloodstream for eventual deposit in growing feather follicles where they crystallize. Carotenoids are deposited only in the cortex, never in the feather core. Cardinals acquire orange, red, and yellow pigments from many seed sources, continuously keeping red plumes vibrant. A caged cardinal fed carotenoid-free seeds would lose its brilliance with successive molts.
Yellow goldfinches, likewise, get their bright color exclusively from the seeds they eat. The inner core of a yellow feather is devoid of pigment, so only the light yellow color of the carotenoid is visible. In many ways, mixing shades of carotenoids is like mixing paints. A Cedar Waxwing’s yellow tail-band can turn orange through consumption of red fruit. Similarly, orioles can become redder and tanagers can become very orange.
The pigments are processed in the liver, but the bird’s appearance doesn’t change until new feathers grow. For many songbirds, deeper colors in the food source make stronger colors in the feather. Behavioral scientists suggest that the brightness of carotenoid-colored plumage indicates a male bird’s physical condition and influences mate selection.
Carotenoids may be acquired indirectly at the end of a food chain. Brine shrimp feed on algae containing red and yellow carotenoid pigments. The pigments eventually cycle into the feathers of birds that eat the shrimp, such as flamingoes and Roseate Spoonbills. Captive flamingoes and spoonbills fade to white without carotenoid-rich dietary supplements, so zoos must feed pigment extracts to maintain the bright coloration of their pink birds.
It would seem to follow that green birds would acquire color from eating green plants, but no matter how much chlorophyllous plant material a bird eats, green pigments will not transfer to its feathers. Most green feathers are the result of a blue structural color overlaid with yellow cortical pigment. It is one of the most complex colors birds produce.
The porphyrins, one of the less common groups of pigments, also produce green feathers. One type of green porphyrin was thought to occur only in the turacos, a family of African birds related to our cuckoos. But pigments with similar spectral qualities have been found in the green feathers of the Jacana, the Blood Pheasant, and the Crested Wood-Partridge. In addition, porphyrins create red feathers in turacos and reddish-brown feathers in goatsuckers, bustards, and owls. Parrots are the only birds with psittacin pigments. Also known as psittacofulvins, these pigments produce vivid red, orange, and yellow plumes, including, for example, the red feathers of Scarlet Macaws. Carotenoids derived from fruits, nuts, and seeds are present in the bloodstream of parrots, but their feathers are colored with psittacofulvins – pigments that they internally “manufacture.” A parrot’s diet, therefore, doesn’t control its plumage colors.
In the 17th and 18th centuries, physicist Sir Isaac Newton was interested in light and color. He was among the first to suggest that small structures within feathers were responsible for color and iridescence in peacocks and other birds, and he was right. Feather structures create iridescent and noniridescent colors.
Why grackles are iridescent
Iridescent feathers are made up of complicated arrays of minute reflectors, often in the barbules, and sometimes comprise several ordered layers of melanin granules, air cavities, or both. The structures are sometimes twisted, so they work like the crystals of an infinitesimally small chandelier, absorbing and reflecting a range of wavelengths.
In male peacock feathers, reflectors are spaced and shaped to reflect different wavelengths, producing an array of hues. Similar processes create iridescence in grackles, hummingbirds, and other shiny-feathered birds. The effect enhances some wavelengths and cancels out others. Iridescent birds are shiniest on bright days when the sunlight’s intensity is high.
The noniridescent colors arising from feather structures are white, blue, and occasionally green. White feathers do not have carotenoid or melanin pigments. All the light that strikes them is reflected, like a white cloud. Swans are white all year, and ptarmigan are white only in winter.
White is also the color of some of the most unusual birds: albinos. Albinism specifically describes the absence of melanin. White birds with melanin pigments in their eyes and skin are known as leucistic, not albino. An albino squirrel, like an albino crow, is white and has pink eyes because it lacks all traces of melanin. But an albino Red-winged Blackbird has white feathers, pink eyes, and normal, bright patches of red and yellow on its wings – colors produced by carotenoids. Another less common plumage abnormality is melanism, in which excess melanin occurs, producing black feathers that normally would be another color.
Knowing something about feather color dynamics makes birdwatching more interesting. You see every bird as a color puzzle with elements of chemical and structural color in its plumage. The feathers of the familiar pet bird Budgerigar, for example, contain only black and yellow pigments, yet Budgies come in white, yellow, blue, and green. Why? Absence of pigment makes them white; cortical pigments make them yellow; core melanins combined with a reflective cloudy zone make them blue. Cortical yellow pigment combined with structural blue produces green.
Armed with the details of feather color, we are free to ponder questions that would be strange to the uninitiated – with answers that are even stranger. How would each of our backyard birds appear if their carotenoid supply waned? Would the bird on the sunny lawn be the same color in dark shade beneath trees? How would an albino Barn Swallow appear? An albino Green Jay? An albino Yellow-shafted Northern Flicker?
Julie Feinstein is collection manager for the new Ambrose Monell Collection for Molecular and Microbial Research at the American Museum of Natural History in New York City. Previously, she managed the museum’s molecular systematics laboratory in the Department of Ornithology. She has master’s degrees in botany and tropical ecology and is currently working on a PhD in biology. Her published papers include reports on bird systematics, and she has birded on five continents.
What makes feathers blue?
Unlike virtually every other feather color, no pigment turns feathers blue. We’ve known that for decades. Instead, it’s long been thought that a layer of cells on blue birds’ feathers reflected light at blue wavelengths, similar to the phenomenon that makes the sky blue. Now, however, scientists have another explanation.
To find the source of blue color, we have to drill down past the barbs and barbules to the “nanostructural” level of the cloudy zone. Nanostructure refers to a size range between molecular and microscopic, a size close to the length of light waves. Nanostructures are measured in nanometers. We know them indirectly by studying their effects.
In the cloudy zone of blue feathers, the melanin and air cavities are so close that the distance between them is shorter than a wavelength of light, according to research by Richard O. Prum of Yale University and his colleagues. When scattering elements are this small, they interact with light through a process called constructive interference. The nanostructural array in blue feathers scatters light in an orderly way. The scattered light waves are in phase and reinforce each other.
You can explore the uniqueness of blue feathers by observing a feather in different light conditions. In the shade, a Blue Jay’s feather will appear gray because the intrinsic gray-brown color of the melanin pigments is visible. Looking through the feather to the light, you’ll see just the gray-brown color of melanin as light passes through the feather. When you place it so that light falls directly upon it and is reflected to your eye, the feather will appear blue. Try the same with a cardinal’s feather and it will be red every time.