In The Solitary Reaper, the poet or speaker bears the song in his heart, 'long after it was heard no more,' because the beauty of the song transcends time. Despite not knowing the theme of the Highland lass' song, the poet tells us that her singing is so beautiful that it exceeds the allure of a nightingale and cuckoo-bird's songs.
In the poem, the poet imagines that the girl may be singing about some old sorrow, or battles of long ago, or mundane dilemma, or perhaps, some private grief which she has experienced and may experience again. The melody and eternal significance of her song appears to speak of universal longings, and the poet finds himself transfixed by her song. Because her song transcends time and space, the poet tells us that he will carry her song in his heart long after it is heard no more.