This fall is one of the mildest in recent memory and many bears are still out of their dens actively foraging. Mild fall weather and late den entry are not completely related, however. Black bears in Maine enter their winter dens as early as late September and as late as mid December. The most influential factor that drives bears into their dens is the lack of natural food abundance. During really poor food years like 1995 in Maine, many bears entered their dens in mid to late September. It is all determined by the bear’s energy balance. That is, when a bear is expending more energy foraging than it consumes for a period of time, their body changes, they lose their appetite and prepare a den for the winter. Weather is also a factor in the energy balance equation as it can increase energy expenditure during foraging helping to send the bear into a negative energy balance. Merely cold weather has very little impact on bears energy expenditures because they are so well insulated against cold. Deep snow is another story, dramatically increasing the energy expense of foraging.
Bear dens are highly varied. Most bear dens in northern Maine are well protected cavities. Many of these cavities are the result of partially uprooted trees. Other common cavities include hollow trees or logs and rock crevices. Some bears dig an excavation in the ground or expand a small opening to spend the winter in. In central and southern Maine, where snow depths are less extreme, many of the bears choose to spend the winter in open ground nests. These are often built in softwood thickets out of spruce and fir boughs and resemble a big bird’s nest on the ground with no protection from rain and snow. Wherever they den, bears commonly collect materials from the forest floor and surrounding trees such as leaves, bark, or boughs to make a bed inside the den. Extended thaws and rain events during the winter can cause some dens to become temporarily flooded with water. While the adults can easily withstand such an inconvenience, these floods can be deadly to newborn cubs. Cubs are born small and nearly hairless (about the size of a red squirrel) around mid January. Cubs that are less than 2 weeks old are too undeveloped to crawl up on their mother to escape the cold water.
