Where do the wild things go?


By Michele S. Byers

Winter is coming and wildlife is getting ready. New Jersey’s wild animals – mammals, birds, reptiles, amphibians and more – have many strategies for coping with winter. Some hibernate, some migrate and others stay put.

Who does what?

• Hibernators – Hibernation is more than a long winter’s nap. Animals that hibernate – including bats, woodchucks, bears and snakes – employ an amazing variety of physiological strategies.

Over-wintering bats and large rodents like woodchucks are New Jersey’s true hibernators. These mammals slow their metabolism to a torpid, cold, inactive state. Woodchucks spend many months with a constant body temperature of just 38 degrees Fahrenheit. Almost never would a woodchuck wake up as early as Groundhog Day on Feb. 2, unless they are living in Punxsutawney, Pa.

If these true hibernators wake up in mid-winter, their metabolism and body temperatures go up and cost them a loss of energy. That is why white-nose fungus has taken such a heavy toll on our smallest bats. The fungus disrupts their sleep and causes them to wake up and fly, burning up precious fat reserves needed to get them through winter.

Black bears are famous for hibernating and like to stuff themselves into cramped places like caves, rock crevices and hollowed-out trees. But bears don’t drop their body temperature much. They are too big to allow their bodies to get really cold and they need to be able to wake up quickly in an emergency. They enter a state of low metabolic activity and can recycle proteins so they don’t have to urinate.

The hibernation-like state used by cold-blooded reptiles and amphibians is called “brumation.” While hibernating mammals are in a deep sleep, brumating creatures move around on warmer winter days.

One interesting example is the timber rattlesnake. In northern New Jersey, they brumate in deep, rocky mountain crevices with southern exposure. But in the Pine Barrens, they brumate in cedar or maple swamps beneath the roots of old trees, where the water never quite freezes.

Amphibians and turtles also brumate. They bury themselves in mud at the bottom of ponds and swamps, surviving in extremely low oxygen conditions. Fish enter a torpid state as water gets cold, but can also become active when the water warms.

Body chemistry helps other creatures survive the cold. Mourning cloak butterflies, for example, have “anti-freeze” compounds in their body fluids, which enable them to survive the winter inside hollow trees.

• Migration – Many of New Jersey’s summer songbirds are “neo-tropical migrants,” meaning they fly south to spend their winters where insects and nectar are abundant in the neo-tropics of Central America and South America. Among the New Jersey migratory birds wintering in the south are warblers, hummingbirds, thrushes and vireos.

Monarch butterflies are unique among insects in that they also migrate long distances, from New Jersey to central Mexico.  As they gather by millions in the pine forests above Mexico City, they respond to colder temperatures by engaging in “muscular shivering” to raise their body temperatures.

While many creatures migrate out of New Jersey for the winter, some migrate in. For example, seals come south from New England in search of a more plentiful supply of mackerel, herring and squid. Seals generally arrive in December and stay until spring, congregating in especially large numbers at Sandy Hook and Barnegat Light.

Tundra swans migrate into New Jersey looking for open water in the Pine Barrens and along the coast. Other birds that migrate to New Jersey from points north include snowy owls, loons, snow geese, northern gannets and bufflehead ducks.

• Year-Round Residents – This state we’re in has many mammals – like deer, foxes, coyotes and squirrels – that stay active all year round. They survive the cold by increasing their body fat during warm weather and growing a thick coat of fur.

Squirrels may be the ultimate survivalists, spending most of the fall collecting acorns and pine cones and stashing them away. On especially cold days in winter, they will huddle together in nests for shared warmth.

Deer also gather together for warmth in winter, often seeking out sheltered areas like groves of conifers. They conserve energy by not moving as much. When snow prevents them from reaching grass and plants, they will eat twigs and bark.

Foxes and coyotes, which feed largely on small rodents, don’t change their behavior much in winter. Their thick insulating winter coats allow them to hunt and sleep in relative comfort.

Chipmunks, raccoons and skunks don’t hibernate, but are far less active in cold weather. They enter a state of torpor, with a lower body temperature and heart rate – more like napping than a deep sleep.

Birds that remain in New Jersey for the winter are those that eat mostly seeds, insect larvae and dried berries. Our year-round birds include American goldfinches, cardinals, chickadees, blue jays, titmice, woodpeckers and many species of sparrows and finches.

Nature is interesting all year round, but get out and explore this winter and you may be surprised at what you find.

Michele S. Byers is the executive director of the New Jersey Conservation Foundation.