For Wordsworth, the selection of the two birds helps to convey the "other worldly" quality of the solitary reaper's song. The song that the woman sings inspires the speaker, presumably Wordsworth, to think of a world far removed from where he is. The song transports him to a realm where time and space are suspended. In this realm, the birds of the nightingale and the cuckoo help to convey this domain of being in another world. Both birds are not standard in terms of being seen every day. The image of the nightingale conjures up images of a world beyond what is. The cuckoo bird is equally rare. Both birds are depicted as welcoming and inviting to travelers not of their setting. The nightingale's chaunt is linked to travelers among the Arabian sands, while the Cuckoo bird's song travels as far as "the Hebrides."
For Wordsworth, these birds help to communicate the power and sense of mystery that the song has in his mind as he hears it. These birds' significance is to enhance this feel, something that enables the listener to the song and the reader of the poem to be taken into a world far removed from what is. The use of the birds helps to facilitate this transformation of what is into what can be.
Wordsworth has chosen the songs of nightingale and cuckoo for comparison with the solitary reaper's song because nifghtingale and cuckoo are the two most melodious birds of the word & the poet suggests that the song of solitary reaper was even more sweeter.