At the beginning of the story, the nightingale overhears the young student longing for a red rose to give to the girl he loves. The nightingale declares, "Here at last is a true lover...Night after night have I sung of him, though I knew him not." The implication here is that the nightingale sings about true love. A little later, the nightingale says, "What I sing of he suffers: what is joy to me, to him is pain." The implication here is that the nightingale sings of joy but also—unwittingly—of suffering, too.
In the middle of the story, the nightingale promises the student that he will have his red rose. She will sing with her heart pressed against a thorn and make the red rose out of her own heart's blood. Before she flies away to make this sacrifice, she tells the student that, "Love is wiser than Philosophy, though she is wise, and mightier than Power, though he is mighty." Here then the nightingale suggests that she sings of love that is wise and love that is powerful.
At the end of the story, when the nightingale sings its songs for the red rose, she sings first "of love in the heart of a boy and a girl." One might infer from this quotation that the nightingale sings of young, immature love. Next the nightingale sings "of the birth of passion in the soul of a man and a maid." One might infer from this quotation that the nightingale sings also of passionate, more mature love.
Finally, the nightingale sings of "the Love that is perfected by Death, of the Love that dies not in the tomb." One might infer from this quotation that the nightingale sings of love which is eternal, and of love which overcomes all obstacles. In short, this final song is about love in its fullest, most ideal form.
Unfortunately, at the end of the story, the red rose which the nightingale sings for, and dies for, is tossed into the gutter and forgotten. The nightingale's songs have been for nothing, because, as the student says, love is "quite unpractical, and...in this age to be practical is everything."