The temperatures plummet to single digits, and maybe the winds howl too, and even if the lovely sun is shining, the days are short, food is scarce, and life in the outdoors can be tough. Many land birds just avoid dealing with winter’s cold; when I was a kid (way, way back), we used to sing a silly ditty that went like this: It’s gonna be a l-o-o-o-ng winter/And vhat vill da birdies do den?/De’ll fly to the sout’/vit’ a vorm in der mout’/and tuck der heads under der vings, da poor t’ings. (Why do I remember such foolishment?)
Flying to warmer climes and better food supplies works for our sapsuckers, warblers, swallows, ruby-crowned kinglets, and most of the thrushes (although they don’t carry worms in their bills). Hibernation works for fat-storing bears; beavers stay relatively cozy in their lodges, feeding from their cache of sticks. But our local birds don’t have snug dens in which to lounge about and they don’t hibernate (although the poorwill does, in the Lower 48). Long ago, one of the archbishops of Uppsala, Sweden is reputed to have said (and I no long have the exact quote) that swallows hibernate in lakes, where they cling beak to beak and foot to foot, in frozen balls; they can be collected and put in the oven, where the heat dissolves the balls into swallows. (I hope this gent knew more about the supernatural than he did about the natural world!).
The problems of staying warm enough and getting enough food for energy are closely linked. Wintering birds need to find food to generate body heat (and pay for the costs of foraging); they also have ways of conserving heat, and thus reducing the energy requirements.
Winter-resident birds can lay on a little fat during the fall; they also can put on some fat during a day of foraging, although the energy in that fat is likely to be used up overnight. Small birds, with a high ratio of surface area to volume (heat is lost through the body surfaces but generated in the tissues), need to feed all day long. They are not likely to be able to pick and choose only the items of high energy value, given the short days, but may just fill up as quickly as possible. In fact, I (long ago) did some food-choice experiments with juncos, using an array of seeds having different energy content, and found that the birds favored small, starchy millet seeds over bigger, oil-rich sunflower seeds, preferentially gobbling up the smaller seeds quickly, even at below-freezing temperatures. On the other hand, some studies have shown that nuthatches sometimes do prefer heavier seeds — perhaps because they, but not juncos, commonly store their seeds.
Some of our wintering birds ease the problem of food-finding by caching (and hoping that some other bird does not raid their stored food items). Nuthatches and chickadees hoard seeds and sometimes insects in bark crevices, and even brown creepers sometimes stash bugs in the bark. Our jays, crows, and ravens store bits of meat and other goodies in various places. I recently watched a winter-visiting magpie collect load after load of pet kibble from my deck railing, picking up over thirty items each time, flying off to some dense spruces, and returning for more in only a few minutes, clearly having stashed its collection somewhere in the trees.
Winter birds may also fill up their crops before nightfall and digest the stored food overnight. Bacteria in the digestive tract help digest the food and even generate a little heat in the process. Grouse and ptarmigan, which eat nutrient-poor twigs and foliage in winter, develop a bigger crop and longer intestines in winter, so they house a greater population of gut bacteria. They also have long sacs (called caeca) attached to the gut; the caeca harbor additional digestive bacteria. The greater capacity of the digestive tract lets these birds hold more food overnight, as well as deriving some warmth from all those actively metabolizing bacteria.
Shivering is good way of generating body warmth, and winter birds often shiver all night long, but that, too, costs energy. So, unless the previous day’s foraging was really profitable, a cold night of shivering could easily use up the fat gained during the day and leave the bird extremely hungry in the morning.
However the body heat is generated, it must be conserved. One obvious strategy for conserving heat is finding a sheltered nook for roosting. Golden-crowned kinglets, grouse, and others retreat to dense conifer foliage; ptarmigan burrow into the snow. Chickadees, nuthatches, and brown creepers take shelter in cavities or deep crevices, and the creepers sometimes bundle up together, several snuggled closely, reducing the total surface area and sharing body heat.
The feather coats of birds provide remarkably good insulation; all the tiny, static pockets of air trapped in the fluffy feathers greatly reduce heat loss. A molt in late summer commonly produces a winter coat that is thicker than the summer one, in some cases almost double the thickness. Birds fluff up their feathers in the cold, increase the amount of trapped air and the insulation. One astonishing report found that the difference between a very cold ambient air temperature and the warm temperature inside a kinglet’s feathers could be as much as a hundred and forty degrees Fahrenheit. Heat can be lost readily from uninsulated body parts, which is why roosting birds tuck their bills and heads into their feather coat and squat down closely to cover their bare feet with feathers.
Birds have high basal metabolic rates and high body temperatures (well over a 100 degrees Fahrenheit, significantly higher than humans’ 98.6). That high metabolic rate creates a high demand for energy. That’s a problem when food is hard come by, especially when cold temperatures increase the energy demands. Wintering birds can reduce the problem by a carefully regulated hypothermia or torpor — allowing their body temperatures to drop a few degrees during the night, thus reducing metabolic costs. Although they may also do this in summer, seasonal changes in their physiology allow them to drop their body temperatures more deeply in winter, sometimes as much as 10 or 12 degrees. Naturally, there is an attendant cost of this: raising the body temperature in the morning, after a night of torpor, costs some energy, but presumably less than the amount saved during the night.
• Mary F. Willson is a retired professor of ecology. “On The Trails” appears every Friday. Her essays can be found online at onthetrailsjuneau.wordpress.com.