The direct/descending motor pathway is both necessary and sufficient for normal song production, while the anterior forebrain pathway (AFP)is necessary for song learning but not production. Juvenile birds who have not yet fully learned their songs and that have lesions of the AFP never learn to make normal songs.
The direct/descending motor pathway is crucial for producing songs normally, and it's also enough on its own. The anterior forebrain pathway (AFP), however, is only necessary for learning songs, not actually singing them. Juvenile birds with damage to the AFP can't learn to sing properly.