Bear in winter
Black Bear: In northern Wisconsin, hibernating female black bears wake up just long enough to give birth. © Kent Mason

Stories in Minnesota

Minnesota Nature Notes

It’s time for bears to hibernate and waterfowl to migrate.

Thousands of tundra swans feed and loaf in wetlands during migration.
Tundra Swans: Thousands of tundra swans feed and loaf in wetlands during migration. © James Martin

Winter doesn’t officially begin until the solstice on December 21st, but by November, cold temperatures have already induced many mammals that are not active during winter to begin hibernation. Black bears are the largest mammal in the state to undertake this extended period of dormancy and most bears are “asleep” in their dens by November.

It used to be that bears were not considered true hibernators. Bears can wake-up if disturbed in their dens and female bears (sows) give birth to their cubs during January and February. Sow bears nurse and care for their newborns during hibernation. Is this the same way that a much smaller mammal hibernates, such as a chipmunk?

Chipmunks experience cycles of deep hibernation when their body temperature is dramatically lowered, interspersed with periods of activity (eating stored food, defecating) within their burrows when their body temperature is raised. Bears do not experience these cycles: they are mostly torpid throughout the entire winter (typically until late March or April); not eating, drinking, defecating or urinating for many months. However, bears drop their body temperature much less, allowing them to react more quickly if disturbed.

Bears are extraordinary in that they actually recycle their metabolic waste during hibernation, using it to make new protein—they are the only mammals known to do this. It is this unique ability that allows them to go months without urinating, eating, drinking or defecating. Bears even redeposit calcium onto their bones during hibernation, compensating for the loss of bone mass that would otherwise result from months of inactivity. It is these unusual adaptations in bears for winter survival that have caused many biologists to expand the definition of hibernation to include the seasonal behavior of both bears and small mammals like chipmunks.

Bears are sustained by their fat reserves during their many months of hibernation. When they emerge from their dens in the spring, bears may have lost 25% to 50% of their body weight. The need to build fat reserves to make it through winter causes bears to eat voraciously in the fall, a behavior called hyperphagia when bears gorge themselves on berries, acorns and other foods. If these natural crops are in short supply, hungry bears may become nuisance bears when they wander near homes to raid bird feeders and trash cans or forage in farmers’ fields of corn and sunflowers.

The likelihood of a bear encounter is greatest in the northern two-thirds of the state, where bear habitat is best, such as at the Conservancy’s Upper Manitou Forest Preserve. The black bear population in the state is robust—between 15,000 and 20,000 animals – but bears are wary and secretive making them difficult to see in the wild. Bear sign may be easier to find: look for tracks, droppings, and claw marks on trees.

Birds, of course, don’t hibernate—they migrate, if not overwintering. Waterfowl are perhaps the most conspicuous of the birds that fly through the state on their way south, and impressive concentrations of these birds arrive in November. Thousands of tundra swans gather on the Mississippi River in southeastern Minnesota.

The swans stop here to rest on the way to their wintering grounds on the mid-Atlantic coast, and to take advantage of abundant supplies of their preferred foods: the starchy tubers of water plants such as arrowhead, wild celery and sago pondweed. Several locations along the river south of Wabasha are good spots for swan viewing. The Minnesota DNR recommends the Weaver Bottoms (just south of The Nature Conservancy’s Weaver Dunes Scientific and Natural Area), and Riecks Lake Park, across the river in Alma, Wisconsin, where an observation platform provides easy access to wetlands with swans. Another good location to see swans and other fall waterfowl is south of Brownsville from river overlooks along Highway 26.

Swans aren’t the only waterfowl to visit the state in late fall. Ring-necked ducks gather in large numbers—at times exceeding 100,000 birds—at Rice Lake National Wildlife Refuge. The ducks are most numerous in October but can linger into November at the refuge. Lake Pepin, a natural impoundment of the Mississippi River by Lake City, attracts the largest concentrations in the world of migrating common mergansers, an almost loon-like diving duck that can number in the tens of thousands in November. When watching the mergansers, be sure to also look for bald eagles that perch along the lake shore and often harass the lake’s many ducks.