During the recent sub-zero cold snap, coming just as January was transitioning to February, a pair of crows showed up in the ash tree outside our windows, just as dawn was breaking. Surprisingly, positive identification took a few moments.
At first glance I thought they might be ravens, given their bulky appearance, and knowing that ravens are heavier in the body than crows. Both are found where we live. But unless both are seen together—not rare, but not an everyday occurrence—it can be hard to make the distinction. But it became clear within a few moments observation that they were crows, not ravens.
Watching them as they perched silently in the ash tree, occasionally hopping from branch to branch, it dawned on me why I had at first been uncertain. Every so often they would give their body a vigorous shake, and with each exertion would fluff up their jet-black feathers, appearing “bulked up,” and less sleek than crows normally do.
In doing so, they were employing one of Nature’s simplest and most effective tactics for dealing with extreme cold, a behavior demonstrated by many species of birds that spend winters in very cold climates. Backyard bird watchers are likely to see this behavior, too, as chickadees, juncos, blue jays and other visitors to their feeders and bird-friendly plantings do the same feather-plumping routine. But to what purpose?
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Trapped air is an excellent insulator. Air trapped in the spaces within the down and feathers of even a tiny bird can create a layer of warmth between the skin and the bird’s outer feathers. Heat retained in this way is a metabolic benefit to the bird, increasing its chances of survival by reducing the body’s need for food to balance caloric intake with life’s warmth and energy demands.
Humans take advantage of the same principle in the design and manufacture of garments and other gear when conserving heat is important. Products like jackets, coats, vests and sleeping bags, for instance. The difference being, the feathers that provide the insulating elements in these consumer products are requisitioned from the creatures that grow them; often as a byproduct of raising these birds for food.
Goose and duck down have for many years been the gold standard for insulating clothing and gear that is expected to provide maximum warmth with a minimum of weight. The air trapping ability of down remains long after it has been harvested from the creatures upon whose body it originally grew.
There are still some—purists, they might be called—who consider garments and cold-weather gear that are insulated with down to be the ultimate. It is lighter, and can be compressed more and still bounce back to its original heat-retaining loft—fluffed up just as a bird might, you could say—and, with care, is likely to last longer than synthetics. Prices reflect this, too.
The only real performance drawback of down is that it insulates poorly when wet. But there is now a process by which down can be treated with a water-resistant coating that resists moisture, before being incorporated into a garment or other product.
As in other aspects of life, the cleverness of humans in taking pages from Nature’s notebook is evident in the warmth business, too. There are a number of synthetic materials that have taken the place of natural down for insulating cold weather garments and outdoor gear. In these, fine filaments of various formulations of polyester are spun to create air pockets between the fibers, not so different in effect from the air pockets created when a bird fluffs up its feathers.
In the highest end synthetics, finer filaments and more air space—more warmth-retaining potential—are characteristic. As good as natural down is, high quality synthetics may have a leg up in high-exertion—sweat-producing—activities, and in wet or humid environments, thanks to their ability to pass moisture and to dry more readily.
Next time you see a songbird at your backyard feeder, or in a tree or shrub in the yard, or along a ski trail, pay attention to what it’s doing when it’s not actively feeding. You just might see it employing one of Nature’s most universal strategies for keeping warm during a Minnesota winter.
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