Jeff Rennicke

Nothing. No tracks but my own are stitched into the dusting of fresh snow, white as birch bark, that fell during the night. No flittering shadows in the trees, not a sliver of bird song in the air.

What sun there is this time of year shines weakly, halfheartedly through the white gauze of clouds, offering not even the slightest pretense of warmth. For nearly a week temperatures around my Wisconsin cabin have not risen above zero. The mercury seems painted to the bottom of the thermometer. A shiver runs through me as I stomp my feet for warmth and then listen again for any sign of life. The only sound is from the bare tips of branches chattering like teeth.

At first glance nature doesn't seem to have invested much in this late-winter day. The forest can seem like a rough etching—barren, lifeless and gray. The sight of flakes parachuting into the front lawn, which swept you up in December, now just means you have to scrape your car windshield. There are subtle beauties—pine branches tipped in white, the pale-blue glow of moonlight off the snow. But this deep into winter, you look less for beauty than for signs that spring has not been forgotten.

They are not easy to find. Once it was believed that nature simply wiped the slate clean every winter, a kind of yearly apocalypse followed by a miracle rebirth each spring. Mice were thought to regenerate spontaneously from rag piles. Frogs and turtles climbed out of puddles, spawned by magic spring rains. Birds changed with other animals to get through the frigid months.

The real ways nature copes with the cold are almost as amazing as these old tales. Winter gives wildlife two basic choices: leave or tough it out. In some places, the landscape empties like a jug of water kicked over. Branches bend under the weight of mixed flocks of blackbirds, cowbirds and starlings, a hundred thousand strong, gathering for mass migrations. Two-thirds of the bird species that nest in North America move to warmer climates.

A hundred million monarch butterflies, like wildflowers on wings, travel sometimes 4000 miles to Mexico, Texas and California. Caribou stream out of the high Arctic with the first frosts of winter. Gray whales travel thousands of miles seeking warmth, food and sunlight.

Not all migrations span the globe, however. Many species make short trips, sometimes only a few miles, to take advantage of local conditions known as microclimates. Elk in Colorado move from high country to nearby valleys. Bald eagles in Alaska seek open water. White-tailed deer in these Wisconsin woods search out a south-facing slope to catch the morning sun.

Other creatures devise their own ways to deal with the harsh realities of winter. Musk ox stand with their backs to the below-zero wind, slowly breathing through nostrils that warm the supercooled arctic air before it is taken into the lungs. Polar bears stay warm by laying on layers of fat up to seven inches thick beneath a coat of fur with nearly 10,000 hairs per square inch. Their rough footpads are skid-resistant on the ice.

The survival of some species seems nothing short of miraculous. The chickadee, for example, weighing just one-third of an ounce, seems a tiny spark of life to throw to the mercy of frigid, 40-m. p. h. winds.

To keep their internal furnace stoked, chickadees eat twice as much food in winter as in summer. They feed almost constantly during daylight to accumulate a layer of fat that will burn slowly through the cold night. They also have 30 percent more feathers in the winter and can fluff them up, trapping a layer of warm air.

When it gets very cold, chickadees lower themselves into a kind of controlled hypothermic state, dropping their body temperatures as much as 20 degrees below the normal 104, thereby slowing energy consumption. With any hint of warmth, chickadees emerge from their sheltered caverns of thick brush, chirping softly and eating, always eating.

I cross a small creek. Bending down, I shovel the snow off the surface and tap the ice with my mittened hand, imagining a painted turtle somewhere beneath it half-hearing the thud as it waits patiently for spring.

Somewhere in these woods, too, are the hibernating black bears. Each fall, triggered by some ancient memory of winter, black bears go on a feeding frenzy. They consume up to 20,000 calories a day, adding 30 percent to their body weight. With the first snow, they den—deep in hollow logs, caves, shallow holes lined with grass. Sometimes they den up to 90 feet high in the broken-off trunks of ancient trees. Their heart rates drop to ten heats a minute, and they settle in for four to six months.

They do not eat, drink, urinate or defecate. Research on how they recycle waste without poisoning their systems has helped in treating kidney patients. Learning how they manage long periods of inactivity without calcium loss or atrophied muscles may help prevent osteoporosis and have implications for long-term spaceflight.

The bear's utter faith in the return of spring keeps coming to my mind. Standing here on the thin edge, a few degree from a climate unsuitable for life, it is comforting to know that under the snow, bears are sleeping with an innocent belief that the sun will come again and unlock the rivers and make the flowers bloom.

Just as I start to turn back home I hear it: the soft, two-toned whistle of chickadees. As I search for them, I see a downy woodpecker spiraling up a birch tree, its blaze of reds as sharp as a tongue of flame. On the ground, I notice rabbit tracks where moments ago I had seen only unbroken snow.

These slight signs of life make it possible to believe in spring again. They help me appreciate the beauty of what is left of winter and remind me that the cold won't last forever. Each track, each snippet of bird song, each frozen seed-pod, is an affirmation of life, a defiance to the cold, a promise.

Take heart, they seem to say. Spring is coming soon.