It's not all too different from your old-school color wheel. Feathers face their fair share of damages, too. They accumulate dirt, are exposed to ultraviolet radiation, and get worn down by parasites and microbes.
Perhaps this is why birds spend so much time grooming. Study after study has shown that male birds with well-maintained plumage are healthier, more socially dominant, preferred by more females, and have higher reproductive success than their more haggard counterparts. The substance helps to keep keratin flexible, allowing feathers to stay water-repellent, providing protection against feather-degrading bacteria, and more.
It also makes feathers appear more deeply saturated with color. Flamingos use the carotenoids in their preen wax to make the carotenoids in their feathers appear even more colorful. In a sense, the wax functions as a type of avian make-up.
This sort of cosmetic manipulation has been seen in at least 13 bird families. While most of them make their own ingredients, some use parts of the environment, like soil. Perhaps the best-known example of a bird that uses dirt as make-up is the Lammergeier , a species known for its bright orange underside, neck, and head.
At one time its color was attributed to carotenoids; but now it's widely accepted as being sourced from iron-oxide-rich soils. No one knows why the vultures bathe in filth , though it's evident that the ruddiest ones get the most respect. The final pigment group is comprised of the porphyrins, not responsible for any particular color type. Often several different pigments are mixed to almost infinitely expand the color possibilities.
Whether color is true or structural, of course, makes no difference to either the birds or their admirers. JSTOR is a digital library for scholars, researchers, and students.
By: James MacDonald. June 3, May 31, Share Tweet Email Print. A Turaco in South Africa. Getty Structural color in birds is produced when light hits the structural features of feathers and bends the light waves. Weekly Digest. Have a correction or comment about this article?
In the case of birds, these pigments play another crucial role, since their presence increases the toughness of the feathers. This explains why even those species with an almost pure white plumage like the swans and the albatrosses and petrels display black tips and edges on their wings, as these are the feathers that are most exposed and damaged during flight.
Porphyrins are responsible for the vibrant pinks, reds, browns and greens of many gallinaceous species such as the peacock and pigeons. Psittacofulvin and turacin are pigments that are even more exclusive: the former only appear in the order Psittaciformes birds typical of the tropical regions, like parrots, cockatoos and macaws ; the latter is typical of turacos.
Both pigments are the origin of the intense greens and reds that distinguish these birds. These three types of pigments are exceptional for three reasons: for their bright and intense colours, because they are unique to a few groups of birds and because these birds synthesise them after having developed specific processes. Following these metabolic pathways, the most exotic birds modify the structure of carotenoids, and therefore their colour, to generate porphyrins, psittacofulvins and turacins.
Thus the presence and influence of these special pigments also depends on the availability of foods rich in carotenoids. In addition to pigments, there is another way in which birds colour their plumage. The iridescent plumages that, for example, characterize the throats of hummingbirds are caused by the existence of a type of nanostructure that acts as a prism, breaking down the sunlight, which causes iridescence and makes the appearance vary depending on the angle of view.
There are other types of nanostructures, such tiny sacs or pockets full of air, that disperse part of the incident radiation—in an effect analogous to that which occurs in the atmosphere and which colours the sky blue. These nanopouches are responsible for the plumages that distinguish and provide the name for birds such as the Western bluebird or the Mountain bluebird. The range of colours also increases when the presence of these nanostructures is combined with that of melanin granules.
The appearance of these nanostructures in plumage is not controlled directly at the cellular level. It is an autonomous or self-directed process—technically, an induced self-assembly—by which they are generated in a random manner during the development phase of each feather. And that randomness makes it impossible for them to generate complex patterns or designs.
In , a team of Spanish scientists from the CSIC found that practically all the patterns exhibited by birds in their plumages arise from the presence and activation of melanins. And that repetitive scheme can occur within the same feather or be produced by the combination of adjacent feathers. And only in 53 species—one species of stork, 37 species of tropical pigeons and 15 of cotingas— were these designs not based on melanins.
Some of these reflected light photons are collected and thus seen by an observer's eye, thereby adding color to the perceived image. Because blue light has very short wavelengths, it is reflected more easily than other colors of light with longer wavelengths. This was first understood in , when scientist John Tyndall noted that miniscule particles in earth's atmosphere preferentially scattered blue light resulting in the familiar "sky blue" of a clear summer day.
Shortly afterward, Lord Rayleigh John William Strutt demonstrated that Tyndall's "fine particles" are actually individual gas molecules in Earth's atmosphere, specifically, nitrogen and oxygen. In feathers, preferential scattering of light by small air cavities or keratin particles overlying a dark melanin layer results in blue coloring. The other colors of light are absorbed by the melanin layer, intensifying the color.
Blue plumage color is often referred to as a "Tyndall blue" structural color. Tyndall scattering can be demonstrated at home using a simple experiment to produce a pale Tyndall blue color.
First, mix one or two drops of milk into a glass of water then place this glass in a dark room and focus a flashlight upon it. The fluid will appear bluish. This bluish color results from blue light bouncing off milk particles suspended in the water while other, longer, light wavelengths pass unobstructed through the fluid. Of course, milk has some larger diameter particles in it that also reflect light wavelengths that are slightly longer than blue, thereby contaminating the pure "Tyndall" blue color.
Like the above described milk experiment, blue colors in the plumage of most bird species results from preferential scattering of blue light by feather structure. For example, when a blue feather is observed under a powerful microscope, the surface layer of keratin appears cloudy or milky due to the presence of small air cavities. Keratin is the proteinaceous connective tissue that feathers, hair and nails are comprised of. A cross-section of a feather reveals an underlying layer of melanin granules and tiny air pockets in the middle of the feather barb.
These small air cavities act like tiny particles by selectively scattering blue light while dark-colored melanin granules absorb longer wavelengths of light, intensifying the blue color. In contrast, structural differences are immediately obvious when a red feather, which derives its color from red pigments, is viewed under the same microscope.
The surface of a red feather is transparent and colorless while the underlying structures are filled with red pigment granules that reflect only red light. Further, differences between structural and pigment colors can be easily demonstrated using several simple experiments.
Because structural color is entirely dependent upon reflective structure, a blue feather becomes dark when it is ground into a powder.
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