The Multiple Miracles of Bird Feathers
Other animals fly, make nests, chirp, lay eggs, and do all sorts of things birds do. But only birds have feathers, among the most magical creations in all of nature.
The subtle rust and charcoal hues of the robin's plumage told me it was a female, and her feathers shone fresh and porcelain smooth in the sunlight. She cocked her head, hopped, and then lunged forward to root at something in the soil. Tilting upright again, she suddenly launched skyward, turning sharply around a fence post and swooping up at an impossible angle to land on an alder branch. Perched there, the robin shook her tail and fluffed up her body feathers before letting everything settle back into place. Then she began to preen, turning and dipping her beak to lift and comb individual quills and vanes, like a fussy housekeeper arranging and rearranging the furniture.
On any given day, up to four hundred billion individual birds may be found flying, soaring, swimming, hopping, or otherwise flitting above the earth. That's more than 50 birds for every human being, 800 birds per dog, and at least a half-million birds for every living elephant. It's about four times the number of McDonald's hamburgers that have ever been sold. Like the robin, each of those birds maintains an intricate coat of feathers--roughly one thousand on a ruby-throated hummingbird to more than twenty-five thousand for a tundra swan. Lined up end on end, the feathers of the world would stretch past the moon and past the sun to some more distant celestial body. Their exact number is unknowable, but one thing is certain: From the standpoint of evolution, feathers are a runaway hit.
Animals with backbones, the vertebrates, come in four basic styles: smooth (amphibians), hairy (mammals), scaly (reptiles, fish), or feathered (birds). While the first three body coverings have their virtues, nothing competes with feathers for sheer diversity of form and function. They can be downy soft or stiff as battens, barbed, branched, fringed, fused, flattened, or simple unadorned quills. They range from bristles smaller than a pencil point to the 35-foot breeding plumes of the ongadori, an ornamental Japanese fowl. Feathers can conceal or attract. They can be vibrantly colored without using pigment. They can store water or repel it. They can snap, whistle, hum, vibrate, boom, and whine. They're a near-perfect airfoil and the lightest, most efficient insulation ever discovered.
Standing there, watching my robin, I was hardly the first biologist enthralled by a feather. Natural scientists from Aristotle to Ernst Mayr have marveled at the complexity of feather design and utility, analyzing everything from growth patterns to aerodynamics to the genes that code their proteins. Alfred Russel Wallace called feathers "the masterpiece of nature . . . the perfectest venture imaginable," and Charles Darwin devoted nearly four chapters to them in Descent of Man, his second great treatise on evolution. But the human fascination with feathers runs much deeper than science, touching art, folklore, commerce, romance, religion, and the rhythms of daily life. From tribal clans to modern technocracies, cultures across the globe have adopted feathers as symbols, tools, and ornaments in an array of uses as varied and surprising as anything in nature.
Pick up a feather and run it between your fingers. It feels light and soft, yet sturdy, the hollow quill tapering upward to a graceful vane. Whether it slipped loose from a gull's wing or escaped a down pillow, the design is unmistakable. We know immediately that it came from a bird; nothing else is so uniquely avian. Birds fly, but so do bats and mosquitoes. Birds lay eggs, but so do fish, newts, and crocodiles. Gorillas make nests, crickets chirp, and squid have beaks. Of all the conspicuous traits and behaviors that make a bird a bird, only feathers are theirs alone. So where did they come from? The answer to this question lies deep in the Mesozoic era, closely intertwined with the origin of birds themselves. To answer it, scientists must rely on that rarest of finds: a feather made of stone.
Fossils of any kind are rare--the vast majority of creatures and plants die and rot far away from the accumulations of silt, ash, or other sediments that might preserve them--but feathers are exceedingly scarce. Like skin, hair, or soft body tissues, they face a double challenge on the road to fossilization. They decompose faster than bone or shell, and they're easily damaged by the heat and pressure required to form even the softest mudstone or shale. It's no coincidence that paleontology museums resemble giant bone yards; the hard bits are the ones most likely to survive.