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.
Traditionally, theories about the rise of feathers focused on the question of why they evolved, proposing some particular use as the original driver of their evolution. Insulation, waterproofing, display, and most often flight have all had their adherents, but lack of fossil evidence made these theories impossible to test. New thinking has focused instead on how feathers evolved, identifying the series of developmental steps required to grow something so structurally intricate. Now, informed by a trove of newly discovered fossils in China, most experts agree that the early stages of feather evolution can all be found in the dinosaurs. This allows ideas about the first functions of feathers to be revisited: There are fossils of simple feather bristles that were clearly colored, and early branching plumes had the same downy traits that keep a songbird warm in the middle of a northern winter.
One night at the tail end of an ice storm, while I was studying winter ecology in western Maine, the skies cleared and the temperature plunged to 17 degrees below zero. That's cold enough so that a Budweiser, spilled in the snow, will freeze solid before all the beer can drain from the can. I know this because I dropped one on the walk between the cabin and my tent.
As I readied myself for sleep that night, I couldn't help thinking of the hapless "chechaquo" in Jack London's Yukon tale "To Build a Fire." But where he had hoped to ward off the frost with "mittens, ear-flaps, warm moccasins, and thick socks," I had a plush goose down sleeping bag to burrow into. (I'd borrowed it from a friend who was sleeping indoors beside the woodstove.) Lying there in warmth and comfort, it occurred to me that somewhere nearby, the tiny birds that I had been chasing all afternoon were doing exactly the same thing.
The thought of any creature surviving outdoors in such frigid temperatures is impressive, but the golden-crowned kinglet does it with the smallest body mass of any bird in the north woods. In ecology, Bergmann's Rule states that body size generally increases with latitude--larger species flourish in colder climates because bulky things maintain their temperature more efficiently. Put a big pot of stew out in a snowstorm, and it will stay hot a lot longer than a grilled cheese sandwich or a fried egg. A golden-crowned kinglet, however, weighs in at just more than five grams, about the same as a nickel or a teaspoon of salt. That's less than half the size of the chickadees and nuthatches it flocks with, and where those birds huddle for warmth in empty nest holes at night, kinglets appear to make do in the open air. Shivering is one strategy to generate body heat, and some species can slow their entire metabolism at night, entering a kind of low-temperature torpor to pass the time until sunrise. But clearly only one thing keeps kinglets and countless other birds from freezing solid in the coldest climates: the incredible insulative quality of feathers.
To date, no amount of engineering has created a synthetic insulation as efficient as feathers. By one estimate, a mountain climber would have to wear 11 pairs of polypropylene long johns to achieve the same heat retention as one down-filled expedition jacket. For kinglets and other winter survivors, the effects are even more astonishing. When temperatures dip low, the difference between the outdoor air temperature and the cozy space inside a kinglet's feather coat can be as much as 140 degrees Fahrenheit.
But no matter how remarkable downy insulation may be for songbirds and mountaineers, perhaps nothing about feathers inspires us more than their close association with flight. As the late author Douglas Adams once wrote, "There is an art . . . or rather, a knack to flying. The knack lies in learning how to throw yourself at the ground and miss."
When Ken Franklin jumps from an airplane, he throws Frightful out first. And once he and the peregrine falcon are airborne, Ken drops a lure, a weighted aerodynamic bait, that gives her a target to aim for as she dives. Several years ago Ken, an avid pilot and master falconer, added one more thing to the list of items falling from his plane: a National Geographic film crew. Using a tiny modified flight computer attached to her tail, they clocked Frightful diving in a streamlined free fall at 242 miles per hour, a record for animal flight. And with close-up video footage, they were able to see how she did it, stretching her body into a streamlined shape and accelerating downward to do what Ken calls "slipping through the molecules."
Feathers enhance bird flight in all kinds of important ways, but the complexity of a living bird wing defies easy quantification. "Their flexibility is amazing," Ken said, staring at a dramatic photograph of a falcon pursuing a sandpiper at close quarters. "In all the dives I've watched, I've never seen a feather break in flight. Wrestling on the ground with prey, sure, but never on the wing."
That's an impressive statement considering the tremendous structural strain of braking and turning at such high speeds.
I asked Ken what role feathers played in a falcon's great speed, and he immediately pulled out a picture of a bird in flight. "Look at the feather edges--they're jagged," he said, tracing a finger along the wing coverts and the contour feathers covering the back. The tips did look uneven where they overlapped, as if some of the barbs were especially long and stiff. "It has something to do with airflow, reducing turbulence and drag during their dives. I don't know exactly how it works, but I'm sure that's what is going on."
The flight computer attached to Frightful's tail coverts has revealed some amazing statistics. She once dove after a lure dropped from 3,000 feet, accelerating to 157 miles per hour before neatly catching it and pulling up from her stoop a mere 57 feet above the ground. The gravitational force on her body in that moment has been calculated as high as 27 Gs. Fighter pilots typically lose consciousness at anything over nine.
Before passing out completely, pilots often report a loss of color vision--something unthinkable for a bird. Birds live in a world of vivid color, where the brilliance of their plumes can signal everything from gender and maturity to their prospects as a spouse. Traces of feather color have been found on the earliest known feather fossils, suggesting that the evolutionary impulse to adorn and impress has been around for a long, long time.
For many male birds, their semi-annual molt provides the opportunity for adornment, the chance to don a set of elaborate breeding plumes they will later lose in favor of more cryptic colors to pass the off-season. But even the drabbest birds must molt, growing new feathers to replace those worn thin or damaged by parasites and constant use. It can be a precarious time: Growing new feathers takes a huge investment of energy. For those who raise their own chickens, it's a hard reality: Common barnyard hens molt once a year, a frustrating period when chicken owners find themselves feeding their flock every day, yet buying eggs at the grocery store. The birds can't help it--in the ordered priorities of survival, maintaining one's feathers takes clear precedence over reproduction.
Molting also results in the happy occurrence of discarded plumes, offering a fleeting reminder of the countless ways in which birds and their plumage touch our everyday lives. My wife remembers her grandmother saying, "You're never more than three feet from a spider." Even the best-kept home hosts scores of them, tucked into dark nooks and corners or hiding behind the walls. Well, you're never far from feathers, either. If they're not stuffing your pillows and parkas, they're covering every bird in every forest, field, backyard, suburb, and cityscape. You'll find feathers and their influence in fashion magazines, airplane wings, fishing lures, ballpoint pens, and fine art, but above all, in the birds--those commonest of creatures so casually adorned with miracles.
Adapted from Feathers: The Evolution of a Natural Miracle (Basic Books, 2011)