I think you mean “hibernate”. Birds migrate (well, most do). Bears, on the other hand, eat a crap ton of stuff, find a hole in the ground and go to sleep. It gets cold up here in the winter; so cold that it's more efficient for some animals to sleep through the winter. Hell, people do it too, to a lesser extent. read more
Black bears, Grizzly bears and Brown bears do go into a deep sleep during the winter months, known as torpor. Hibernation is when animals “sleep” through the winter. During this sleep, the animals will not wake up when they hear a loud noise or even if they are moved or touched. read more