Feathers

In 1860 or 1861, a quarryman in Bavaria, Germany, discovered the fossilized impression of a feather preserved in limestone. This feather was 58 millimeters (about 2.3 inches) long and asymmetrical, with the vane on one side of the quill being twice the width of the vane on the other side, and was similar to the flight feathers of modern birds. A year later, a fossilized skeleton of a feathered creature was discovered in a quarry near where the feather had been found. The creature was namedArchaeopteryx lithographica—Archaeopteryx from the Greek words meaning “ancient wing,” andlithographica in reference to the use of limestone in lithography. Archaeopteryx has since been identified as a genus of theropod dinosaurs and is believed to have been a transitional species (or group of species) between reptiles and birds. Because of its feathers, it is generally regarded as the first bird.

Structure and Color of Feathers

Feathers continue to define birds. They are the only living animals that have feathers, and all birds have some type of feathers. Feathers result from a modification of the outer layer of the bird’s skin, the epidermis, and probably evolved from scales as found on reptiles, such as snakes and lizards. They are composed largely of keratin, an inert substance that is light, strong, and long-lasting. While a bird is still developing inside its egg, it is covered with bumps called papillae, and the epidermis folds inward around each of the papillae to form follicles. From these structures feathers are grown and regrown. The number of feathers per bird varies greatly between species, from less than one thousand on a ruby-throated hummingbird to more than twenty-five thousand on a tundra swan.

A typical feather has a central shaft with two vanes arising from opposite sides. On flight feathers, the vane from the leading edge of the feather is narrower and more rigid, thereby maintaining the wing structure and producing the lift necessary for flight. Amazingly, this basic feather structure has remained unchanged since Archaeopteryx flew above the dinosaurs of the late Jurassic period. The portion of the feather shaft within the follicle and beneath the skin is called the calamus. The portion of the shaft above the skin is called the rachis, from which extend numerous opposing parallel branches, called barbs, that collectively form the vanes. Hooked structures called barbules join the barbs together, producing a smooth, sheetlike surface.

Color is imparted to feathers by colored substances called pigments and by variations in feather structure that alter the manner in which light is reflected. There are three different types of pigments found in birds: melanins, carotenoids, and porphyrins. The most common pigments are the melanins, which produce black, a spectrum of brown and reddish-brown shades, and light yellows. Examples include the black of a crow and the yellow of a baby chicken. Birds synthesize melanins from dietary proteins. Carotenoids are produced by plants and then ingested by birds when they eat the plants; these pigments produce red, orange, and yellow feathers, such as the red of the northern cardinal. Porphyrins are metabolic breakdown products produced by birds from hemoglobin, the oxygen-carrying component of red blood cells, when red blood cells become old and are broken down by the liver. They produce a variety of colors, including reds, browns, greens, and pinks, and are responsible for the brown feathers of many owls.

A bird's coloring can be further affected by structural properties of the feathers that change how they reflect incident light. Iridescent colors result from the microscopic barbules causing incoming light to refract, while certain noniridescent colors, such as the blue of bluebirds and blue jays, result from air pockets in the barbs that cause light to scatter. Often the multiple colors of a bird’s feathers result from some combination of both pigment and structure.

Functions of Feathers and Molting

While flight is certainly the most outstanding function of feathers, they serve many important functions, enabling birds to exist and thrive in environments all over the globe. Feathers provide insulation by trapping air and thereby helping birds control their body temperature. The color and form of feathers help birds communicate and signal when they are ready for breeding, and in some cases keep them camouflaged from predators. Adults sitting on a nest of fledgling birds often depend greatly on their feather coats to make them less visible against the background of the environment. Feathers also help birds remain waterproof while swimming or diving. Some feathers help birds remain clean, and others help them support themselves, such as the stiff tail feathers of a woodpecker that act as a strut against the trunk of a tree. Specially modified feathers serve as sense organs, able to detect feather position and movement essential for the complexities of flying.

A fully developed feather is a dead structure; when it becomes worn or broken it must be replaced, as repair is not possible. Molting is the process of replacing all (complete molt) or a portion (partial molt) of the feather coat. This process varies widely among birds in terms of timing, completeness, and number of occurrences per year. However, most adult birds undergo a partial molt prior to breeding season, frequently yielding bright, colorful feathers known as the alternate plumage. A complete molt occurs after the breeding season and results in the basic plumage. A good example of this is the American goldfinch, which sports bright yellow and black feathers in the spring (alternate plumage) before reverting to a drab olive (basic plumage). The complete molt provides some migrating birds with new feathers for the long flights to their home territories. Other birds get new feathers to help insulate them from the winter cold.

The beauty of feathers and the wonder of flight have attracted the attention of humans throughout history. Feathers have adorned priests and warriors as well as the hats of fashionable women. In 1905, Guy Bradley, a warden hired to patrol wading-bird rookeries in southern Florida, was shot and killed by plume hunters. His death, and the resulting public outcry, eventually led the US Congress to pass the Migratory Bird Act of 1913, also known as the Weeks-McLean Act, which banned the hunting of migratory birds as well as the importation of wild bird feathers for use in fashion. It was soon replaced by the Migratory Bird Treaty Act of 1918, which put migratory birds and their parts—including eggs, nests, and feathers—under federal protection.

Principal Terms

keratin: fibrous proteins that are the chemical basis for feathers

molting: in birds, the process of shedding old feathers and replacing them with new ones

pigment: a substance that selectively absorbs and reflects light to appear as a certain color; pigments produced by living organisms impart color to their biological structures

shaft: the long, central spine of a feather

vane: the flat, broad web emerging from opposing sides of a feather shaft

Bibliography

Bostwick, Kimberly. "Feathers and Plumages." Cornell Lab of Ornithology's Handbook of Bird Biology, edited by Irby J. Lovette and John W. Fitzpatrick, 3rd ed., John Wiley & Sons, 2016, pp. 101–48.

Feduccia, Alan. The Origin and Evolution of Birds. 2nd ed., Yale UP, 1999.

Gill, Frank B. Ornithology. 3rd ed., W. H. Freeman, 2007.

"How Birds Make Colorful Feathers." Bird Academy, Cornell Lab of Ornithology, Cornell U, 2015, academy.allaboutbirds.org/how-birds-make-colorful-feathers/. Accessed 5 Oct. 2016.