One of the first snakes to emerge from its long winter hibernation is the Garter Snake.  Although some individuals hibernate singly, others congregate on a specific area.  That area may be an old barn, a rock wall, or a specific hillside.  The requirements for a hibernaculum are not fully understood, but when you find such a place expect there to be snakes there for years to come.  The first warm week of spring at a Garter Snake den is the most exciting.  Males and females emerging often choose this time to mate.  The picture below shows a large female (right) inspecting a small male (left).