Leaving their winter dens

As the days grow longer and the snowmelt gives way to the newly emerging green vegetation, the bears too emerge from their winter sleeping quarters.

Most bears leave their winter dens during a one to two month period during April and May. Both physiological and environmental factors affects why a bear leaves its den and when it does. Physiological factors include the bear's age and sex, its fat reserves and its reproductive status. Environmental factors that influence den emergence include temperature and snow cover, as well as, photoperiod (amount of daylight each day) and the availability of food.

Adult male bears leave their winter dens first, and adult females with newborn cubs leave last - as much as four to six weeks after their male counterparts. The other members of the population, namely, subadult males and females, and adult females with yearlings or two-year old offspring, leave sometime in between.

When bears emerge from their winter dens, many of them may not have eaten for months. During the denning period, bears may lose 15 to 30 percent of their body weight, and a lactating female sow with newborn cubs may lose as much as 40 percent. The greatest nutritional stress on a bear comes one or two months after they leave their dens in the spring. At this time, the animal is no longer in the energy-saving mode of hibernation. The animal is active, so it is burning calories, and food is relatively scarce. For these reasons, bears continue to lose weight well into June and sometimes even into early July.

Most bears remain in the vicinity of their dens for a week or two before they finally leave. Mothers with newborn cubs may spend a month or more in the vicinity of their den. Bears are generally lethargic after the winter, spending the majority of time resting. Biologists often refer to this period as a state of walking hibernation. Cubs, however, are very active when they emerge, exploring their new world with eagerness and enthusiasm.
In early spring, bears will build day beds near their den out of conifer needles and debris raked from the forest floor into a mound. Throughout this period of acclimation, bears shift from energy-conserving metabolism of winter to the energy-burning metabolism of summer. They are sluggish and eat and drink very little.

The bears most vulnerable during April and May are the yearlings and subadults with marginal fat reserves. These animals often exhaust their fat deposits during the denning period, and by the time they leave their den they have few energy reserves; some may even starve to death.

Spring brings plenty of new vegetation - grasses, new leaves and skunk cabbage. The problem for bears is they are not able to properly digest the food that is available. The bulk of nutrients in a mature plant's leaves and stem is locked inside its cells, each of which is surrounded by a protective wall of cellulose, which is indigestible by bears. Because a bear is a carnivore, it has a relatively short intestinal tract and is actually poorly equipped to feed on vegetation, eventhough it constitutes the bulk of their diet.

Bears overcome this handicap by consuming mass quantities and by being selective in what they eat. Often they choose only the parts of the plant that contain the least amount of indigestible cellulose, such as the flowers, seeds, fruits, bulbs, and roots. Bears will eat the leaves and stem of a plant when it first emerges, when it contains the least cellulose and is easier to digest. Bears often follow the snowmelt up the mountain sides in search of newly emerging vegetation.

Once bears leave the den area, they scatter in search of food. Mothers with cubs, however, don't usually venture too far. During the spring, mother black bears will tree their cubs while they go off and search for food, sometimes wondering some distance away, for three or four hours at a time.

May and June are a heavy grazing period for bears. Bears will generally remain along the valley bottoms and throughout low elevation slopes until the snowpack recedes. Bears are often sighted at dusk and dawn, feeding in swamps and grassland clearings; and sometimes in residential areas. Spring is an important time for bears - it is the time they re-learn the locations of good food sources. We must be careful not to teach bears that humans and human's backyards are good sources of non-natural foods. Particular attention should be paid to improperly stored garbage, bird feeders, pet food kept outside, dirty barbecues, and composting activities, all of which attract bears into the community. (For more information see Managing Backyard Attractants).