The Multiple Miracles of Bird Feathers
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.”