As the season progresses toward winter, a bear's diet becomes more varied. The last remaining berries and fish are sought, as are live and hunter killed deer, moose, elk, and caribou. This is the time that bears are trying to put on the final deposits of fat before their long winter naps. In late fall and early winter bears move into their denning areas and begin preparing a suitable den. Black bears usually den in holes excavated under large trees or rock outcrops, or a small natural cavities.
Brown bears tend to dig their dens in steep alpine areas.
Dens are just large enough for bears to squeeze into. Bears rarely eat, drink, urinate or defecate while they are denning. They sleep deeply, but do not truly hibernate, and they can be awakened by loud noises or disturbances. Cubs are born in the den, usually around January. Black bear cubs usually stay with their mothers for a year and a half, and brown bear cubs stay with mom for 2.5 to 3.5 years.