These birds rarely sing the same thing twice.
Song sparrows learn as chicks to improvise on old standards.
Songs stored there can be reeled off at the proper moment. More complicated songs presumably use more complicated templates. The biologist is trying to find out. Nowicki documented song sparrows' variations by making sound recordings in the Pennsylvania woods and observing how birds behaved as they sang and listened to one another's songs. Another theory has to do with attracting a mate.