When the weather becomes extremely cold, many of the animals become dormant or less active. If you look outside on a really cold day you will hardly see any wildlife except for maybe some birds. Many of the animals have found refuge and the optimal refuge is inside the attic of a warm home. Granted it can get very cold in the attic, there is plenty of insulation to bundle up in. Also in attics there are several instances where animals are able to find heat. This could be from the furnace running at maximum capacity, an area where the drywall has been cut to run lighting into the house or maybe there is heating being released from a light that the electrical is run from the attic.
The most common of attic dwellers in the winter cold are grey squirrels, mice and flying squirrels. Sometimes there could be multiple types of animals living in the attic. Rodents especially will use the restroom throughout the attic and also mark the attic with certain scents to mark the attic as a place of refuge or a nursery in the winter time. On rare occasions you will have bats overwintering in the attic but is highly unlikely in the Charlotte area. Typically as you move into warmer climates you will see this and sometimes up near the Great Lakes you will find a colony of bats overwintering in an attic.
Once the weather begins to warm and the winds begin to change direction, many animals will leave there refuge to search for food and mates. It is this transition period between winter and spring that many babies are born and many problems such as squirrels in the attic that may have been going on for years will be discovered.