Wild creatures adapt to winter in a variety of ways, depending upon their metabolism, food requirements and natural insulation. Once the first freeze of autumn descends on the Heartland, most adult insects die off, their species sustained in the form of eggs, pupae and larvae (aquatic or terrestrial). Some species (certain bees, harvestmen, ladybird beetles) overwinter as adults, retreating to protected sites (hives, dens, buildings, leaf litter, mulch piles).
Amphibians and turtles spend the winter buried in soil or pond muck while snakes and lizards retreat to underground dens. Birds, having both mobility and natural insulation, make choices based on their food requirements. Those that feed on flying insects and nectar migrate to southern climes while most seed and berry lovers manage just fine in the Temperate Zone winter; nevertheless, some of these hardy species move to lower elevations or roam about to escape heavy snows. Some birds, known as irruptives, invade the Midwest on an erratic basis, driven south by food shortages (nuts, berries, lemmings) in their northern habitats.
Mammals adapt to winter in a number of ways. Some (bats, ground squirrels, woodchucks, marmots and most bears) hibernate through the lean months, nourished by a layer of brown fat. Others (tree squirrels, chipmunks, pikas, muskrats, beaver) survive the winter by feeding on stored food supplies. Large herbivores (elk, caribou, bighorn sheep, moose) either migrate to sheltered areas or change their diet to more accessible vegetation; they are, of course, followed by the wolves, coyotes and mountain lions that prey on them. Smaller mammals (raccoons, opossums, cottontails, mice) may den up during periods of extreme winter weather but must become opportunists to survive, taking what nature provides (or what man discards).