The thermometer on my front deck read 4 degrees Fahrenheit one morning. As daylight slowly crept over Thunder Mountain, juncos and chickadees began to come to the feeders. The chickadees whisked in and out in the blink of an eye, carrying off a sunflower seed to eat or store in the shelter of a nearby spruce. Juncos, gobbling millet seeds on the flat feeders, behaved very differently. They puffed out their feathers until they looked quite round, and crouched low to cover their bare legs, so much so that they seemed to have no legs at all.
Millet seed is not very full of fat or energy, but that is one of the things juncos consume in quantity. Long ago, I conducted some experiments with captive seed-eating birds, including juncos, studying whether or not they selected more energy-rich seeds if the birds were kept at low temperatures. The answer was no. Instead, they just ate more of the same kinds of seed they typically preferred at higher temperatures. So for these birds, it seems that switching foods is not the usual means of getting enough energy to keep warm.
Bird metabolic rates and body temperatures (104-107 F) are generally slightly higher than those of mammals. A high metabolic rate requires energy. Small birds have greater problems with cold than larger birds because they have more surface area and heat loss in proportion to volume of heat being generated by muscles and organs. Because of this they lose heat more quickly. Small birds usually can't lay down heavy layers of insulating fat because that is incompatible with flying. But some may put down just enough fat during the day to last through the following night.
The simple observations of the junco got me thinking about other wintering birds, and how they deal with cold winter weather. We've all seen birds with their feathers fluffed out, thus trapping more air and insulating their bodies. When they go to roost in the evening, they often find a sheltered spot, for example, on a perch surrounded by dense spruce or hemlock twigs that help reduce heat loss, or, in the case of chickadees, in tree cavities. At least some small birds save nocturnal energy by reducing their metabolic rate at night (a controlled hypothermia), and they generate some heat by shivering as the temperature drops.
Those are the ordinary, conventional solutions to cold weather. Some birds go farther. Grouse (including ptarmigan), for instance, have winter plumage almost double the thickness of summer plumage: body feathers in winter have a well-developed aftershaft - a sizable downy offshoot of the main feather shaft. Thus the body has, in effect, a double layer of feathers, which traps air and provides good insulation.
Ptarmigan go one better, their winter plumage is white. This not only provides protective coloration against a background of snow, but it also improves insulation. White feathers are hollow, and the hollow spaces are air pockets that help keep the cold at bay. In addition, their legs and feet are heavily feathered like a kind of insulated snowshoes. Ptarmigan commonly burrow into snowbanks for overnight bivouacs where they have a snow blanket and protection from wind.
Because grouse are bigger than most songbirds, their surface to volume ratio is better for heat conservation (more volume relative to surface area). They can store some fat and fill a crop with food for later digestion. Most grouse feed on buds and twigs in winter, and this food source is seldom completely covered by snow. Although this diet is usually readily available, it is not very digestible, so grouse have to eat a lot of it. Consequently, their digestive tract gets bigger in winter to accommodate this greater volume of food. In addition, some grouse apparently have the capacity to fast for two or three days, if necessary, living on stored fat and crop contents.
At the other end of the spectrum are tiny songbirds such as golden-crowned kinglets. Weighing only 5-6 grams (less than _ of an ounce), kinglets can't store much fat, and they have to subsist on tiny frozen insects winkled out of winter buds and bark, somehow finding enough to sustain them during the long nights. They'll huddle together at night (as chickadees also do). Two birds together reduce heat loss by about 23 percent and three birds in a bundle can reduce heat loss by 37 percent. But that's not enough. They still have to seek shelter (e.g., under snow-covered branches) and probably need to allow body temperatures to drop several degrees. Even so, overwinter mortality is apparently quite high among bird in this category.
Mary F. Willson is a retired professor of ecology.
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