To what does Wordsworth compare the lass with in his poem "The Solitary Reaper?"

Wordsworth compares the highland lass with nightingales and cuckoos, both of which have been regarded, since ages, to be among the most melodious singers of nature.

The highland lass’ song has had an overwhelming effect on the poet's mind. Though he’s not able to make out anything of her song, he listens “motionless and still” and makes sure that nobody interrupts her singing.

The wistful song touches the poet deep within his heart. He is sure that he’s never heard anything like the “melancholy strain,” flowing out of the mouth of the simple country girl.

Across civilizations and countries, nightingales and cuckoos have always been hailed as the most mellifluous singers of nature. So, we see that the poet picks up the best singers of nature to compare the girl with.

The melody of a nightingale’s song is always soothing to ears and mind. However, it would certainly sound much more pleasing to somebody when he's travelling in hot “Arabian sands” than when he's sitting snugly in his bedroom.

The poet doesn't choose a nightingale’s normal song to compare with the maiden’s song. Instead, he chooses that particular song of the nightingale which comforts the “weary bands of travellers” in a hot desert. Still, the girl outdoes the bird in singing, according to the poet.

The poet further says that the “thrilling” voice of a cuckoo, heard in the spring, too, can’t match the girl’s “plaintive numbers.”

So, we see that not only does Wordsworth compare the lass with the greatest singers of nature, but also declares her to be a better singer than each of them.