Laura Erickson's For the Birds

Now that we’re in the middle of what is passing for winter this year, birds are wearing their thickest coat of feathers. On average, songbirds such as chickadees and crows have between 2,000 and 4,000 feathers, 30–40 percent on the head and neck. House Sparrows have roughly 3,100 or 3,200 feathers during summer, but 3,500 or 3,600 feathers in winter—better than an 11 percent increase.

The increase isn’t in the feathers we can actually see. Those on the outside of a bird are called the contour feathers and provide very little insulation. They’re like the outer shell of a warm jacket, designed to keep the wind out. The down feathers—the ones that actually insulate—are between the skin and the outer shell, where insulation on a jacket is. These soft, delicate structures can be fluffed up by special skin muscles unique to birds. A bird can appear much smaller in summer than on a cold wintry day thanks to its fluffy down.

Of course, winter is the season when nutrients are most scarce, when most wild birds would be sorely stressed if they needed to grow many feathers. The thick down jacket is added during the fall, when food is most abundant and the bird is most able to expend the resources for new feather growth.

Although the black plumage of crows seems especially useful in winter because it absorbs so much solar heat, it also releases the heat rapidly to the air in any kind of wind, so except on calm days, crows have no real advantage over other birds in staying warm. Ravens of the Sahara Desert take advantage of their black plumage to increase their convective heat loss. Even the slightest breeze draws heat from their feathers, allowing them to keep as cool as the black robes and tents of Saharan Desert human tribes. In the American Southwest, the extreme dry air permits huge temperature extremes between night and day. That’s why roadrunners have bare black skin patches on their shoulders—on cool mornings, they raise the outer feathers that normally cover those spots and orient themselves so that the sun shines on the black skin to directly warm themselves.

Air pockets are the key to good insulation, but to stay comfortable, that warm air must be trapped inside some kind of waterproof shell layer. That’s what the tightly-closed contour feathers of cold-weather birds provide. Most chickens can survive fairly cold temperatures, but one mutant hen that could never survive in the wild is the frizzled chicken, so-called because instead of fitting together, its outer feathers curl upwards, allowing rain in and letting body warmth escape as fast as it can be produced. Thanks to this bizarre plumage, these frizzled birds have much higher metabolic rates than normal chickens. In the 1700s, the common belief was that these odd birds hatched the wrong way out of the shell, and because of their frizzled appearance, which seemed somehow stylish but impractical, English people disdainfully called them “French hens.” So if you ever decide to give your true love three French hens for Christmas or any other winter occasion, make sure you cover them in a lot of wrapping paper.