Birdsongs are both innate and learned. however, young birds must hear an adult male sing the species song. why?

The reason for that is the song gets imprinted on the young birds. Have in mind that birds rised  alone will sing but the local pattern will not influence the singing of that bird. So if the birds listen an adult male sing they will help the young ones to get their singing as they will serve as the closest reference. Still have in mind that young birds that are alone can also sing but in a different way, not the one proposed by the environment.