(Editor's Note: This article was originally published on December 26, 2008. Your comments are welcome, but please be aware that authors of previously published articles may not be able to promptly respond to new questions or comments.)
When winter comes to the northern regions, or the south in the lower hemisphere, some animals decide to skip the whole season and sleep through it all until spring. These include the cold-blooded creatures: snakes in their dens, toads in their burrows, frogs in the mud at the bottom of the pond. Birds in general do not hibernate.
Mammals that hibernate include ground squirrels, bats, skunks, woodchucks, and of course bears. Most hibernators are small animals, such as rodents, whose small size makes it more difficult to retain heat in the cold season. Typically, animals go underground to hibernate where the temperatures are less subject to variation, although some, such as raccoons, may decide to den up in a convenient hollow tree. Hibernating bats remain in their caves.
Hibernation involves slowing the animal's metabolism and thus reducing its temperature; in such a state, its energy needs are greatly reduced. Some creatures can live through the winter solely on the reserves of fat they have accumulated in the fall. Many others rouse periodically to feed from caches of seeds or other food they have stored in their dens.
I am using the term "hibernation" loosely. Scientists tend to distinguish between true hibernation and a state of torpor, in which the animal's metabolism is at a higher level. There are disputes as to whether bears, for example, can properly be said to hibernate. Raccoons are another species that does not truly hibernate; in cold climates, they rest in a torpid state during frigid weather while occasionally leaving their dens to forage if a warm spell comes along. In general, though, we will not see most hibernating creatures until they finally emerge in the spring.
Other animals cope with the arrival of winter by getting away from it all. Among mammals, most winter migrators tend to be the large ungulates who travel in herds, such as bison, elk, and caribou. Sometimes predators, notably wolves, may follow the herds. Otherwise, winter migration is largely an adaptation of many species of birds.
Residents of northern North America are familiar with the sight of birds flying south in the fall. In November here in northern Illinois, the air is frequently filled with the cries of migrating geese, even when the skeins are so high overhead that they are almost invisible. Of course north and south are relative. As the residents of Texas, Florida and Mexico welcome the robins and goldfinches to their winter range, the juncos are settling into the neighborhoods that those birds have just vacated. The bird feeders should adjust for the change in clients.
Not everyone seems to get the message that it's time to move on for the season. Looking out my window on a frigid, snowy December morning, I spotted a couple of robins among the starlings on the ground. I can't help thinking they'll be sorry, once it's too late. At the ponds and creeks, mallard ducks are abundant as well, even when the water has frozen. I tend to think this is the bad influence of the Canada geese, who have been overwintering in this area for decades now. Or it could be the general trend to warmer winters. There is also speculation that changes in farming practices have contributed to more mallards remaining in the north for the winter.
If some migratory waterfowl are overwintering in the north, they join the numerous year-round residents. These are the creatures we see all winter long, as well as those who migrate into our area from even colder climates. To survive the winter, animals have to overcome both the cold and the relative shortage of food, given that most plants have died or gone dormant for the season.
For birds, the essential element is generally food. Songbirds in particular, whose small body mass makes it harder for them to conserve heat despite the excellent insulating value of their feathers, need a constant supply of food to keep their metabolism high enough that they do not freeze to death. Many elements of winter, in addition to the cold, make it difficult for them. The days are shorter, leaving less time to forage. Insects, on which many birds prefer to feed, may not be found. For these birds, a feeder may sometimes make the difference between life and death, and certainly the bird feeder provides one of the best possible sites for watching winter wildlife.
Adapting their behavior is the key to survival for many species. While insects are the preferred food of starlings, they switch in the winter to a diet primarily of seeds. Cardinals in the breeding season are highly territorial, and the males challenge any interlopers who cross the line, but in winter, they flock together around the bird feeder like social birds. This gives more individuals access to food, and it may help give warning of the arrival of predators. Chickadees in the fall will cache insects and seeds, which they will then retrieve when no other food is available. Flocks of these birds may sometimes huddle together to share their warmth against a freezing night.
Some birds will also change the color of their feathers from their more conspicuous breeding plumage; the male goldfinch in his winter range can hardly be recognized as the brightly-colored bird of spring and summer, but he is less noticeable to predators. The white ptarmigan of winter is a perfect example of this adaptation.
The predators must also adapt to changing circumstances as their prey either disappears or modifies its behavior. Sharp-eyed raptors learn to spot smaller creatures against the white background of a snowfield, and that bird feeders attract large numbers of prey. For the vulnerable species, the feeder represents a harsh choice between food and safety.
Rodents such as mice and voles are also attracted to bird feeders, where they have the opportunity to scavenge dropped seeds from the ground. Unlike ground squirrels and
other hibernators, these rodents are active throughout the winter. For these overwintering mammals, shelter is of great importance, both for reasons of warmth and safety. Voles live in nests underground, and in winter they construct a network of snow tunnels, where they can move around protected both from predators and the harshness of the wind and cold. Mice will nest in burrows underground, in abandoned bird nests, or anywhere shelter presents itself, including human houses, where they find themselves unwelcome. Voles and mice will also stash seeds in their nests. Because these small mammals are nocturnal, people may not often see them, but in the fresh snow, you can see from their tracks where they have been.
These rodents, along with rabbits, can do a great deal of damage during the winter, as they gnaw on the roots and bark of young trees. Gardeners should be careful not to mound up mulch near the trunks of trees, as it can attract a colony of voles that will damage the roots. Vulnerable young trees should be protected with plastic or wire mesh.
Squirrels, notoriously unwanted visitors to bird feeders, build nests of leaves, known as "dreys," in the upper branches of trees, where they shelter during the worst weather. [top photo] I often find myself cursing at these animals as they dig up my lawn in the fall, caching food for the coming winter. The animals also pack on as much fat as they can during this season, as many mammals do. Squirrels will also invade human houses if they discover an opening to the attic.
Beavers construct sturdy lodges where they live year-round, but they do not hibernate, emerging at night throughout the winter to feed. I have never seen the beaver who built this lodge, but I can always tell where they have been working. An underwater tunnel leads from the lodge to the deep water behind a nearby dammed-up creek. A pack of coyotes has tried but failed in the past to break into the lodge.
Another animal that can cause destruction in winter to shrubs and plantings is the white-tailed deer. Deer gather into herds for the winter and prefer to shelter from the cold in the forest, but a shortage of forage or an increase in the number of deer in the herd beyond the region's carrying capacity may drive the animals to seek food near human habitations. Rabbits also can harm shrubs and trees, as their primary food source in winter seems to be woody plants. After they made it clear how much they love the taste of tender blueberry twigs, I keep my bushes enclosed in chickenwire cages throughout the winter, to keep them from being grazed to the ground. Rabbits do not rely on special winter shelters, remaining instead in the same cover they use throughout the year, such as a brushpile. For a rabbit, safety from predators is the primary use of cover.
Predators have their own problems surviving over the winter, when many of their usual prey creatures are not available. Foxes and coyotes typically prey on rabbits and small rodents, such as voles and mice, and they are regular visitors to the bird feeder, where they know these rodents are likely to be foraging. They are opportunistic feeders and when hungry there is little they won't consume, including small pets left outside. Cats hunting the mice at the bird feeder may find themselves surprised by a predator higher up the food chain than themselves. Coyotes may be hard to sight, at least when a camera is at hand, but there is no difficulty seeing where they have been, from their scat and their tracks through the snow.
If you are interested in wildlife, winter provides one of the best opportunities to indirectly observe the creatures around your home by watching for the tracks they leave in the newly-fallen snow. You can see the trail of the rabbit, the tracks of the mice going to and from the bird feeder, the squirrels coming and going from their trees. You can see if the deer have invaded your yard, or if the coyotes have come hunting. Sometimes a torn shred of fur or a splash of blood in the snow is a sign that a life-and-death struggle has occurred while you were asleep or away. A fresh snowfall is like a blank canvas on which we can see the wildlife in their daily work of survival over the winter. Step outside one snowy morning and take a look!