Your attic is warm, out of the weather, and almost predator proof. They can just add leaves and call it home. Pet food left unattended, or an open garbage can, or a full bird feeder is a free meal to any hungry squirrel. Gray squirrels breed throughout the year, but there are two distinct peaks, in the winter and summer. They prefer cavities in trees (or homes), and when cavities aren’t available, they will build a nest out of leaves and twigs high in a tree.