Nature Notes

Looking out the window at our cold, snow-blanketed landscape, perhaps you’ve glimpsed – or heard – a chickadee (Massachusetts’ state bird), or spotted a gray or red squirrel. Did you wonder how New England’s wildlife manages to survive winter? How do they stay alive?

These two feathered and furry friends stay active through the winter, taking shelter at times in tree holes or dense evergreens. They need to find food on a daily basis so their bodies have fuel for activity and warmth. Back in the fall, responding to the shortening daylight and other cues, their bodies began preparing for winter. The chickadee molted, growing a new, denser set of feathers. The squirrel grew a thicker winter coat of fur. Did you notice that both creatures look fluffier in the cold? They fluff feathers or fur to better trap body heat.

As in so many furry animals, the squirrel’s winter coat has two layers: long, hollow guard hairs covering shorter, denser, soft underfur. Under their skin, furry animals also put on a layer of fat. It serves as added insulation as well as potential fuel for energy and warmth. In porcupines, their hollow, air-filled quills add extra insulation to this slow-moving animal’s coat. (Look up outside, and you might spy a porcupine up a tree.)

The amount of fat varies in different animals. Opossums, on one hand, are especially susceptible to frostbite because their fur coat and their fat layer are relatively thin. Black bears, on the other hand, fatten up so much in the fall that their weight increases by 50 percent or more. They live off this fat through months of hibernation, even as pregnant females develop, bear, and nurse young.

The chickadee too has layers in its coat of feathers. Tightly constructed outer feathers cover downy feathers, trapping heat. Chickadees also have a fat layer, but their amount of fat varies throughout the day as the bird feeds and flits around. Other small birds such as juncos and titmice put on more fat, but birds can’t get too fat. They need to stay light enough to fly.

In fall’s season of plenty, both the squirrel and the chickadee busily searched for food. They hid, or “cached” it for later use. Gray squirrels (and blue jays!) are famous for burying acorns. Red squirrels stockpile nuts and seeds in burrows under a log or tree root. Chickadees store seeds and even insects in bark cracks and crevices. These animals all have good memories for where they’ve stored food. A chickadee’s brain actually grows up to 30 percent larger in the fall as the bird caches food. Its brain then shrinks in spring when those stored-food memories are no longer needed.

There’s more to the story. Chickadees, for example, don’t always find enough food or have enough body fat to get through a frigid winter night. When night-time starvation looms, a surprising energy-saving trick comes to their rescue. It’s like adjusting a thermometer. Chickadees can lower their own body temperature almost 20 degrees F and go into a sluggish state known as torpor. It’s sometimes called controlled hypothermia. Their body processes (metabolism) slow so they need less fuel. These birds can still fly in an emergency. As morning approaches, they shiver to warm up.

A few other birds can also enter overnight torpor, including golden-crowned kinglets (which come here in winter from farther north) and hummingbirds (which go south for winter but are so active that they still may risk overnight starvation in warmer places). Many local birds, however, can lower their body temperature a few degrees at night, without going into torpor.

Some furry animals that spend much of the winter in burrows or dens may also lower their body temperatures, but just a few degrees. They sleep for periods of time – days or even weeks – but awaken to eat from time to time.

Hibernation is a somewhat different matter. Animals known to hibernate, such as black bears, groundhogs (woodchucks), chipmunks, and even bats, go into prolonged torpor. Bears’ body processes slow, although their body temperature drops only about 12 degrees, to 88 degrees F, and cycles. They breathe only once or twice a minute (every 45 seconds), and their heart beats only 8-12 times a minute (down from 40-50 beats). They may awaken briefly. Female bears awaken to give birth and nurse cubs. Chipmunks can drop their body temperature to 43 degrees F and their heart rate to 4 beats per minute. Yet they rouse periodically to eat nuts stored in their underground tunnels.

In contrast, some small animals live energetic, full lives under the snow. When there’s at least 8-12 inches of snow on the ground, heat from the earth melts the snow close to the ground, creating a small space where the temperature remains just a degree or so above 32 degrees F. Decomposers – soil bacteria and fungi – remain active, decomposing leaf litter, creating some heat. Voles make tunnels, chambers, and runways in snow-covered grassy fields. As long as the protective snow lasts, voles, shrews, and mice thrive and even reproduce in this hidden and temporary “sub-nivean” (under snow) environment.

What about animals that seem to disappear in winter – amphibians, reptiles, fish, even insects? They don’t have fur or feathers. Rather, they have thin skin, scales, or exoskeletons. They’re “cold blooded” because their bodies do not make heat to warm them. Instead, their body temperature matches the temperature of their surroundings. They warm up or cool down by moving to warmer or cooler places. To survive stressful conditions, they may go dormant.

Certain frogs, such as bullfrogs and green frogs, reptiles such as painted and snapping turtles, and northern water snakes, avoid freezing by going to the bottom of a pond, or even into its muddy bottom. They become mostly inactive. Their breathing, heartbeat, and body processes slow. The pond water freezes from the top down, and ice expands and floats. The ice actually protects the water and the life below from frigid air and cold winds. The water stays around 39 degrees F. Fish get similar protection. As ice fishers will tell you, the fish below the ice are very much alive.

Frogs, toads, and snakes, such as garter snakes, that live mostly on land often shelter in old animal burrows or tunnels. Some bury themselves in leaf litter or mud and become inactive. American toads spend the winter below the frost line (3 feet underground) in animal tunnels. Wood frogs, gray treefrogs, and spring peepers spend the winter partly or entirely frozen. That’s thanks to sugary antifreeze chemicals that their bodies produce as soon as ice begins to form on their skin.

Finally, what about insects? Many insects pass the winter as dormant eggs. But various insects overwinter in other life stages. For example, in winter, have you seen woolly bear caterpillars (which later become Isabella tiger moths)? These caterpillars develop antifreeze chemicals.

About 75 percent of insects have four life stages: egg, larva (caterpillar), pupa (chrysalis or cocoon), & adult. Other insects (10-15 percent) have three: egg, nymph, & adult. Butterflies in the Westborough area overwinter in different stages. For example, banded hairstreaks overwinter as eggs; Baltimore checkerspots as partly grown caterpillars (larvae); cabbage whites and tiger swallowtails as chrysalises (pupae); mourning cloaks as adults, tucked into a crack.

What about deer ticks (black-legged ticks), which spread Lyme disease? They spend their first winter as larvae, and their second winter as adults. Snow often shelters them, rather than killing them (as we’d like to think).

Does it still seem amazing that wildlife survives nature’s empty, snowy landscape? Keep watching! Soon, creatures will be appearing, ready for food and, for some, their breeding season. Skunks may be among the first you notice.