Winter is a great time to observe the apartment complexes created by pileated woodpeckers in standing, dead trees, a.k.a. snags. These woodpeckers create rectangular holes as they are excavating for insects and they also create cavities to roost in overnight. However, they don't stay in the same cavity each night, because they might not be nearby, leaving some vacancies.
Small resident songbirds such as tufted titmice, chicadees, kinglets, and nuthatches group together in mixed species flocks to find food during the winter months. They stay in these groups overnight in holes in dead trees, sometimes using the vacant woodpecker roosts. The small birds have such high metabolisms, they would use up all their fat and possibly freeze if they didn't huddle together all night. They switch positions like Emporer penguins since the middle most spot is the warmest. They may stay put if there is a storm.
All day long the songbirds search for enough food to keep them warm for the next night. Woodpeckers therefore, unwittingly create warm places to stay for many small animals that might otherwise freeze to death during cold New England winter nights.