Deer herds migrate within a fairly small area to forage for food where the snow may not be as deep and they can get to the grass below. That's why you can often see deer closer to populated areas in winter. Unfortunately, areas can only provide a certain amount of food to feed a certain amount of deer in the winter months, and many die of starvation or freeze to death due to being too skinny to survive. Conservationists and game wardens use that information to determine hunting laws. That's why different hunting areas have different hunting limits. It actually helps keep deer populations more healthy and strong so they can survive the harshness of winter.
Most deer up here in Wisconsin migrate to Mexico or somewhere in Central America. That is one reason why deer hunters in November cannot kill any deer here in Wisconsin. As they migrate a good number of them are killed by cars along the way.