In the fashion world of birds, anything goes. From crimson cardinals to loons in elegant black and white, from portly turkeys with rusty fan-tails to sky-diving falcons in steely gray, birds carry it off with panache. The diversity of birds is amazing, with 10,000 species worldwide living in habitats as different as tropical jungles and frozen tundra, and ranging in size from the tiniest hummingbirds to condors with ten-foot wingspans. Birds’ fashions may seem exotic, but they are also functional, adaptations for each bird’s particular environment and way of life.
A bird’s body differs from other vertebrates – animals with backbones – in ways that are largely related to flight. Birds have a streamlined shape, hollow bones, and light-weight beaks rather than heavy jaws and teeth. They walk on two feet, have wings instead of forelegs, and they sport the lightest, warmest, most versatile body-covering known in the animal world – feathers. Among all the living animals on earth, feathers are found only on birds.
Feathers come in many styles, from sleek peacock plumes to fluffy eider down. Whatever their form, all feathers are made of keratin, a protein similar to that in our fingernails and hair. A typical wing feather is long and narrow, with a central shaft and a smooth, flat web or vane on either side. The shaft is both light-weight and strong, hollow where it attaches to the skin (the quill) and grooved where it supports the vane. Branching out from the shaft are fine projections called barbs that form the vane. Each barb is fringed along its sides with tinier branches called barbules. The barbules have microscopic hooks and ridges (barbicels) that interlock with those on neighboring barbules like hook and loop fasteners. Birds preen their feathers with their beaks, separating the barbs to clean them and zipping them together again to form a smooth yet flexible surface over which air can flow easily.
Feathers have different forms for different functions. Flight feathers are generally the bird’s longest and stiffest feathers, found on wings and tail. Most flight feathers are asymmetrical with a narrower vane on the leading edge and wider vane on the trailing edge. Their aerodynamic shape helps to give the lift and thrust a bird needs to fly. Wing feathers attach to the birds forearm and hand bones. When the wing is folded, they stack up like window blinds or fold like a fan, depending on their location.
Downy feathers have short, flexible shafts, delicate barbs and barbules, and they lack the hooks found in flight feathers so cannot be zipped into a smooth vane. Instead they form a fluffy layer close to the body, a bird version of long johns. Contour feathers are usually downy near the base, with a smooth vane towards the tip. These body-covering feathers grow in areas called “tracts,” with bare skin between them. Arranged in overlapping rows like shingles on a roof, they cover every part of the skin. Many contour feathers have a secondary “after-shaft” that is downy, contributing to the fluffy insulating layer. Contour feathers give the bird protection from the elements, a streamlined shape, and its patterns and coloration.
Some birds sport brilliant colors or striking patterns, while others go for camouflage. Crimson cardinals, golden goldfinches, bright orange orioles, and sky-blue bluebirds are common sights in our woods and fields. Other birds like woodcocks, sparrows, owls, and the females of many songbirds wear dull brown stripes and muted tones that help them hide. Colors like black, browns, reds and yellows come from pigments in the feathers. Often these come from the bird’s food, like the pink of flamingos from their diet of crustaceans. Other colors, called “structural,” are a kind of iridescence, resulting from the refraction of light by tiny pits, grooves and pockets in the barbs. The startling blue of a bluebird and glowing ruby throat of a hummingbird are only seen when light hits the feathers from a certain angle; when backlit, these feathers look black.
Feathers play many roles in a bird’s life, and a bird can control their position for different needs. A turkey fans its tail to impress a female, a red-winged blackbird spreads his crimson epaulettes in territorial defense, and a chickadee puffs out its feathers for warmth. A bird in flight holds the wing feathers together on the downstroke for force and open on the upstroke to let the air slide through. Muscles and nerves in the skin allow a bird to lift, lower, and tilt its feathers at will.
The number of feathers ranges from nearly 1,000 in hummingbirds to over 25,000 in swans, with waterfowl tending to have the greatest number. Birds spend a lot of time preening their feathers to clean them, remove parasites and spread oil from a gland near the tail for waterproofing. Even so, feathers are not living tissue and they wear out, so birds molt and grow new ones once or in some cases twice a year.
Watching the birds in our backyards, it’s easy to see that they don’t all fly the same way. Hawks hunt from the air, gliding on broad wings, using rising columns of warm air to hold them aloft. Falcons and swallows have narrow, tapered wings for making quick turns as they chase flying prey. Grassland birds with short, broad wings can lift straight up from the ground. Hummingbirds are hovering champions, their tiny wings and special shoulder joints allowing them to move up or down, backwards or forwards like tiny helicopters.
Having bodies specialized for flight means that birds lack forelegs and jaws with teeth that other vertebrates use for getting food, grooming, caring for the young, and other essential tasks. Here’s where beaks and feet come into play, adapted and modified in a great many ways, for different functions in each bird’s life.
To get their food, some birds use their beaks and others use their feet. Beaks function like tools or utensils, from strainers to tongs to scoops to pry-bars. Kinglets and chickadees with tiny, sharp beaks probe for insects hiding in bark crevices, and shorebirds with long, slender bills probe for food in mudflats. A wading heron stabs a fish with its sharp bill, and a grosbeak cracks seeds with its stocky bill. Some beaks are very specialized, like the crossbill’s lever-action bill for prying open cones, and hummingbirds’ long, thin bills, for sipping nectar in deep flowers. Birds with a varied diet, like crows and jays, have multi-purpose bills for a variety of foods. Raptors, birds of prey like hawks and owls, have sharp, curved bills for slicing and tearing food, but they don’t use their beaks to catch their meal. Instead, they catch prey with their feet, holding it with needle-sharp talons and strong toes. All birds have a special muscular organ in their digestive tract, the gizzard, for grinding and mashing their food, a good substitute for teeth.
Among birds, high heels are the fashion, for all birds walk on their toes, with the ankle bend partway up the leg. The knee, closer to the body and usually obscured by feathers, bends the same way as ours. Most birds have four toes, the first toe or “hallux” pointing to the rear and the other three pointing forward. Songbirds’ perching feet curl around branches, a special locking tendon holding them in place. When ready to fly, the bird lifts it body, which releases the tendon. Ducks and geese have paddling feet with webbing between the toes, and wading birds like herons have long, wide-spreading toes that distribute their weight over the mud. Raptors have grabbing feet with curved talons, while ground-feeding birds like turkeys and grouse have long toes and claws for scratching up food. Woodpeckers have two toes pointing forward and two back for stability while climbing on tree trunks or chipping out holes, their stiff tail feathers adding extra support as they work.
Among the larger terrestrial animals, birds outshine all others in the beauty and variety of their colors and forms. With the same basic wardrobe, birds have evolved a great diversity of shapes and sizes, plumages and behaviors. Whether flamboyant or muted, ocean-faring or desert-dwelling, predator or prey, every bird’s beak, feet, color, and shape are adapted to its way of life, custom-tailored to its particular needs.
Suggested Reading
Sibley, David Allen. The Sibley Guide to Bird Life and Behavior. New York: Alfred A. Knopf, 2001.
Sibley, David Allen. Sibley’s Birding Basics. New York: Alfred A. Knopf, 2002.
Peterson, Roger Tory. A Field Guide to Birds of Eastern and Central North America. 5th Ed. Boston: Houghton Mifflin, 2002.
Nearby Nature 