A bushy-tailed fox patrols the edge of a snowy field; a blue jay’s call rings out in the frosty stillness; tiny mice prints, like stitching on a quilt, crisscross the snow. These sights and sounds tell of the many birds and mammals that stay active throughout the winter. Like us, these animals must find ways to stay warm in order to survive this season of cold, inhospitable weather.
It’s easy to show why warm-blooded animals face the problem of heat loss in winter. Leave a cup of hot cocoa on the kitchen counter and it will soon cool down until it reaches room temperature. In summer a dish of ice cream will warm up until it melts. This is because heat tends to equalize until everything in one space is the same temperature. On a winter day, a warm-blooded animal will constantly lose heat to the cold outdoors. Avoiding heat loss and generating body heat are two challenges that animals face in winter, and getting enough food is a third – though they are all connected.
Warm-blooded animals produce body heat through metabolizing the food that they eat. However, in winter, a time when animals need to eat more to stay warm, food is usually scarcer and often hidden under a layer of snow. For some the solution is migration. Many birds fly south to spend the winter in warmer climates. Caribou herds migrate south as well, while mountain sheep move to lower elevations and deer to sheltered areas called deer yards. Some animals, like woodchucks and jumping mice, hibernate in winter, lowering their rate of metabolism in order to conserve energy. Others, like chipmunks, raccoons and skunks, become dormant, sleeping for long periods of time and only waking up occasionally to eat. Chipmunks wake up to feed on seeds stored in their underground burrows, while raccoons and skunks leave their dens every few weeks to search for food above ground, returning to their snug quarters to continue their winter naps.
Staying active through the winter is the strategy of a surprising number of warm-blooded animals. The chickadees at our feeders, owls gliding over snowy fields, and woodpeckers hammering on hollow snags are some of the many birds that remain in the north, amazing us with their ability to survive outside in the coldest weather. Many kinds of mammals also keep active in winter, like deer, coyotes, foxes, rabbits, weasels, and mice, to name a few. These animals have special physiological mechanisms that help them prepare for winter and endure its hardships.
Just as we put on layered clothing when we go outside in cold weather, many animals grow special coats for winter. Furry animals like deer and beavers grow a layer of short dense hairs close to the skin. Air is trapped in the tiny spaces between the hairs and warmed by the animal’s body heat. These tiny pockets of still air surround the animal with a blanket of warmth that slows heat loss to the cold outside. In white-tailed deer, the hairs in the outer coat are hollow and crinkled so that air is also trapped inside these tiny spaces. Mammals groom their fur to keep it clean and free of parasites, but also to oil it. The oils help the outer coat to shed water like a raincoat, keeping the undercoat dry and warm.
It’s no accident that we choose goose down as fill for our warmest winter parkas. Downy feathers with their finely fringed barbs are excellent insulators, trapping warmth in the minute spaces of this fluffy layer. Some birds like chickadees grow additional feathers in the fall for extra protection from the cold. Puffy chickadees haven’t put on weight from eating all your birdseed; they’re just fluffing up their feathers to trap air between the barbs for extra warmth. Like mammals, birds have oil glands for waterproofing. Preening with their beaks, they spread the oil on their outer feathers, so they will shed water and keep the downy undercoat dry.
Besides a furred or feathered coat, animals depend on a layer of fat beneath the skin for added insulation. This fat layer serves as a good insulator because it has few capillaries to carry heat out to the surface of the skin where it would be lost to the cold. The blubber of arctic mammals like whales, seals, and even polar bears is a thick layer of fat that provides extra insulation in this frigid region. Fat is also an important source of energy to fuel muscle action, and some animals like beavers and moles store extra fat in their tails that can be metabolized for energy when food becomes scarce. Many birds put on fat before migration to supply fuel for their long flights.
A special kind of fat called brown fat is present in dormant and hibernating animals. Metabolizing brown fat creates heat in a process called non-shivering thermogenesis, warming the animal without the use of its muscles. Hibernating and dormant animals put on weight all summer and fall, for these reserves will be needed to keep them warm through the winter.
Body fat is a good back-up emergency reserve, but getting food on a regular basis is still essential for active mammals and birds. Some animals like squirrels, mice, and chickadees store food for winter, but others like rabbits and deer rely on what they can find each day. Predators like coyotes and fishers will gorge when they make a kill and then fast until the next successful hunt. Many animals change their diets in winter, broadening their menu to include foods not eaten in summer. Whatever the food supply, active animals must eat to stay warm and make it through the winter.
No matter how well-insulated or warmly dressed, most animals seek out shelter in the coldest weather for the additional protection from wind, cold, and wetness that it provides. Tiny birds like kinglets or chickadees find cozy nooks inside snags or among evergreen boughs. Beaver families build massive dens of branches and mud while muskrats build smaller ones of grasses. Flying squirrels huddle in hollow trees, often in large groups, and rabbits seek shelter in brush piles. The snow cover insulates the ground and affects how deeply the frost will penetrate. Some insectivores like moles and shrews stay active underground, seeking their prey beneath the frost line.
Many animals find a surprisingly cozy home in or under the snow. Fluffy, freshly fallen snow is a good insulator and provides protection from wind as well. Ruffed grouse dive into the snow at dusk to spend the night in a hollow warmed by their body heat. Red squirrels burrow through the snow to access their stores of pine cones. Voles and mice tunnel under the snow to the subnivean, a space that forms just above the ground as the snow recedes away from the warm earth. This region offers shelter from cold and wind and access to food caches stored in the fall. It also provides some protection from predators, though owls can still hear mice under the snow, and the female short-tailed weasel’s slim build allows her to follow her prey right down into their tunnels. In the spring the maze of tunnels created by these subnivean survivors comes to light.
Although winter is a difficult time, the many warm-blooded animals that stay active are well adapted to meet the challenge. With fur, feathers, and fat helping them to stay warm, they are adept at finding sufficient food to meet their energy needs and shelter for protection from the elements. Both above and below the snow, many animals go about their lives undaunted by the frosty winter world.
Heinrich, Bernd. Winter World: The Ingenuity of Animal Survival. New York: Harper Collins, 2003.
Holland, Mary. Naturally Curious. North Pomfret, Vermont: Trafalgar Square Books, 2010.
Marchand, Peter. Life in the Cold: An Introduction to Winter Ecology. Third Edition. Hanover, NH: University Press of New England,1996.
Stokes, Donald. A Guide to Nature in Winter. Boston: Little, Brown, 1976.