Berries and hazelnuts become scarce. Bears who are eating acorns continue to feed and fatten. Other bears begin losing weight from expending energy to look for digestible food, or perhaps traveling back to their home range to hibernate. The bear's fur more than doubles in insulative value during the fall in preparation for hibernation. Some bears enter dens in September, especially as their digestible foods disappear.
Most bears enter dens.
Bears now spend some of their precious energy by preparing a different den each year, even when their previous dens would work. They may choose burrows, caves, rock crevices, hollow trees, or depressions under fallen trees or brush piles. These locations provide insulation and also protection from hunters and predators. The entrances are just large enough for the bear to squeeze through, opening into a chamber about 2.5 to 5 feet wide and 2 to 3 feet high. For extra insulation, bears sleep on a nest of leaves, grass and other material that they rake into the den.