Because this particular poem is celebrating the relation of a farmer maiden to her endeavors, connected with the rhythms of Nature, and strengthening the relationship to Nature that she has (a frequent worry of the Romantic, that we are losing our connection, due to industrialization, etc.), these two birds are appropriate symbols, for these reasons: 1, they are solitary birds, not flock birds; 2. their melodious and plaintive song is heard in Nature as distinct voices, like the Solitary Reaper’s; 3. these are the very birds Wordsworth hears as he walks. The opening stanza makes this comparison with the solitary reaper herself, singing “by herself”. Because she is singing in a different language (Erse, a Gaelic language) he ask the same question as he asks of the birds he hears on his walks through the Lake District: “Will no one tell me what she sings?” The last couplet speaks to the memory of the nightingale’s and cuckoo’s songs, which stay in Wordsworth’s memory long after the songs fade away (he stretches his experiences to the Hebrides and to the Arabian sands, to underline the point that these solitary sounds haunt all cultures.)
wordsworth has chosen the song of nightingale and the cuckoo,for comparision with solitary reapers sond because the nightingale and the cuckoo have a melodious voice and the poet says that the girl's song has a soothing effect on the tired travellers.