Illustration by Adelaide Tyrol
Conventional wisdom says that if you put up a bird feeder on Nov. 1 and take it down on April 1, you won’t have a problem with marauding bears, because they hibernate between those dates. After a bear’s visit in mid-November two years ago, I assumed put-up date should be rescheduled to late November. A month later, almost on New Year’s Day, I was astonished to hear that a large bear had just been seen on a porch a few miles from my home in Thetford Center, VT.
What gives? Why aren’t bears in bed by then?
Pregnant females are first to hibernate, reports Ben Kilham, of Lyme, NH, a wildlife researcher, who has studied black bears extensively in the Northeast. He says they don’t begin slumbering until around the second week of December. Next, he says, are the unbred females, who start hibernating around Christmas. The hibernation schedule of adult male bears is harder to predict, because they generally stay active as long as food is available, or until forced to their dens by heavy snow or plunging temperatures.
Kilham says young males often are the last to retire because they have the greatest incentive to keep feeding—so they can grow to compete with mature males. In our area young males can still be active until mid-January.
When they do call it a winter, where do they go? Many people probably envision bears hibernating in rocky caves. Bears certainly use caves or rock crevices if they exist within their home territories. But many bears here must look to other places for winter shelter. They excavate dens under tree roots, or make their own protected cavities beneath tangles of fallen boughs. They spend time collecting leaves, branches and grasses to help insulate their winter quarters. Sometimes the female and her cubs from the previous winter work on the den as a team.
For some reason a bear builds a new den each year even if the old one remains in fine condition. They will build anew even if the den offers less protection. Such was the case in Michigan not long ago, when a female bear spurned her cave of the previous winter, built a new den above ground, but then fell prey, with cubs, to wolves.
Contrary to another popular belief, a hibernating bear is not comatose; it can be roused from sleep in mid-winter in several minutes. Nor does its body temperature drop substantially as is the case with smaller mammals, which must awaken at times during winter to eat and move about. A thick coat of hair and body fat allows a hibernating black bear to maintain a temperature of about 88 degrees F., about eight degrees lower than its summer temperature.
Female bears are likely to become at least somewhat active in winter. They give birth in late January or early February, and they clean and nurse their cubs. Their 1 to 3 cubs are hairless and weigh less than a pound when born, but they weigh 3-to-4 pounds by the time they leave the den, which seems impressive given the fact the mother doesn’t eat or drink over winter. (Nor does she defecate or urinate.) Females may weigh more than 200 pounds before winter, but by spring they are likely to have lost 60 or 70 of those pounds.
Black bears in our region mate in June, and the females must put on as much fat as possible over the following six months to assure survival of both her and offspring. However, fertilized eggs do not implant in the female’s uterus and become embryos until November or December. This process of implantation apparently is disrupted if she hasn’t put on enough fat—which would help assure her survival, without cubs, over winter. This would explain why bear birth rates drop dramatically after lean summers.
To build fat, bears might travel miles in a single day to forage for berries, nuts, apples and other woodland plants. They eat much and sleep little.
Black bears in our region face a special challenge: the growing fragmentation of forests by housing, commercial developments and highways, which can cut off their access to natural feeding areas and tempt them to try other food, such as sunflower seed. Not all homeowners with birdfeeders are as tolerant of bears as those in the Thetford neighborhood where I live. The last thing a homeowner wants is a bear coming to his or her house for food.
“A fed bear is a dead bear,” is an old saying that too often becomes a sad reality.
Li Shen is an adjunct professor at Dartmouth Medical School and a member of the Thetford conservation commission.