Bird feathers: Flight, Warmth, Color and Courtship
What comes from a bird, weighs very little, and flutters instead of traveling far when tossed? A feather is the answer: air resistance quickly slows its broad, light surface, while bird feathers attached to wings and tails help living birds control their flight.
Feathers grow from tiny follicles in the outer skin and are made of keratin proteins. A typical vaned feather has a central shaft called the rachis, with branches called barbs on either side. Each barb divides into smaller barbules, whose tiny hooks connect neighboring branches into a continuous surface. At the base, the shaft widens into a hollow tube called the calamus, embedded in a skin follicle. The vane does not extend onto this lower section. Worn feathers are replaced periodically through molting, with new feathers developing in the same follicles that produced the old ones during an earlier stage of the bird's life. A wing feather combines this branching architecture with stiffness that resists air during the downstroke while allowing flexibility in other directions.
Bird feathers take different forms for different jobs. The vaned contour feathers cover the outside of the body, while fluffy down beneath them traps insulating air. Down lacks the hooks that fasten neighboring barbules together, leaving those branches free to spread. Hairlike filoplumes grow close to contour feathers and are often hidden underneath them. Stiff bristles with few barbs occur around the eyes and bills of some birds. Flight feathers on the wings and tail are especially important for steering and controlling movement through air.
Insulation reaches beyond the adult bird's own body. Feathers can be plucked to line nests and protect eggs and young from cold. Some hatchlings have special natal down that is pushed out as their later plumage emerges. Many birds grow contour feathers in distinct tracts rather than evenly over every patch of skin, with down and filoplumes sometimes growing in the spaces between them. Birds incubating their own eggs often lose feathers from part of the belly, forming a brood patch. Penguins, ratites, and screamers are among the groups whose feather distribution differs from the usual tract pattern.
Water brings another set of demands. Some birds have powder down that grows continuously as tiny particles break from the ends of its barbs. The powder is thought to help with grooming, and powder-down patches shed water, although their precise function remains uncertain and a waterproofing role has been proposed. Herons work it loose and distribute it with their bills, while cockatoos may use their heads as powder puffs. Preening helps maintain waterproof plumage: a bird guides barbules back into their interlocking positions and distributes oil that keeps the feathers healthy, flexible, and able to shed water. The resulting interlocking surface and its careful maintenance help feathers resist water while preserving the smooth vane needed to support flight.
The colors of bird feathers come from pigments, microscopic structures, or both working together. Melanins produce dark shades and browns, while carotenoids contribute reds, yellows, and oranges. Some pigments occur only in particular groups, including distinctive red and green pigments in turacos. Blue colors and iridescence often arise from structures that reflect, scatter, or refract selected wavelengths of light. White feathers lack pigment and scatter light diffusely. The blues and bright greens of many parrots involve light reflected from different structural layers. In green plumage, yellow pigment can combine with structural effects to produce the final visible color.
Those colors serve both concealment and courtship. Different upper and lower colors can camouflage birds during flight, and muted patterns help predators or prey blend into their surroundings. Males and females of many species have strikingly different plumage. A peacock spreads a long train of ornamental feathers bearing eyespots during its courtship display. Male long-tailed widowbirds also carry spectacular breeding plumage: an experiment that shortened or lengthened their tails found greater mating success among birds given elongated tails.
People have put bird feathers to work in clothing, bedding, tools, and decoration. Down from geese and ducks is widely used as insulation, while wild eider down is especially valuable commercially. Feathers have served as arrow fletching and as decorations on fishing lures. Feather waste from poultry production has also been used in biodegradable polymers, enzyme production, and materials for growing microbes.
References
Cornell Lab of Ornithology: How Feathers Are Built — https://academy.allaboutbirds.org/features/all-about-feathers/how-feathers-are-built.php/1000
Weber State University, John Cavitt: Feathers & Feather Tracts — https://faculty.weber.edu/jcavitt/feather1.pdf
Peter Stettenheim: The Integumentary Morphology of Modern Birds — An Overview (abstract) — https://www.researchgate.net/publication/255631933_The_Integumentary_Morphology_of_Modern_Birds-An_Overview
Malte Andersson: Female choice selects for extreme tail length in a widowbird — https://www.nature.com/articles/299818a0
Qingxin Li: Progress in Microbial Degradation of Feather Waste — https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2019.02717/full
- Weber State University, Feather structure (J. Cavitt) — faculty.weber.edu