Singing

Here's a tip: whenever old school poets (like Wordsworth) talk about singing, they're also talking about poetry. That means that all the references to songs in this poem are references to poetry. The solitary reaper is a poet, just like the speaker, and in some ways she's his counterpart. All those little birds? They're poets too, and they also are also the types of birds that poets love to talk about (especially the nightingale). The poem in some ways then is a story about how poets can be found just about everywhere, and in the strangest of places.

• Line 3: The woman is reaping and singing, and she's all alone. She's both a farmer and a poet (her singing symbolizes poetry).
• Line 6: The woman is singing a "melancholy strain." We have no clue why the woman is singing a sad song. She could be sad, or she could just like the song.
• Lines 7-8: The Vale itself is "overflowing" with the sound in a way that almost makes it seem as though it too is singing. The woman's song, and the Vale's echo, symbolizes the close or harmonious relationship between the woman and nature.
• Lines 9-12: The nightingale symbolizes poetry for two reasons. One, it's a bird that sings, and (we all know by now) singing means poetry. Two, it's a bird that poets always mention. Coleridge and Keats are just two of the famous poets who have poems about nightingales.
• Lines 13-16: Like the nightingale, the cuckoo also symbolizes poetry. Unfortunately for the cuckoo and the nightingale, the solitary reaper's song is much better (at least so says our speaker).
• Lines 17-20: The speaker cannot understand the woman's song and wonders if perhaps she is singing about grand, epic themes ("battles long ago"). 
• Lines 21-24: Then again, maybe the solitary reaper is singing about more "familiar" or "humble" matters. The "humble" and the "familiar" actually describe much of Wordsworth's poetry, and in some ways they symbolize the thematic focus of Romantic poetry. 
• Lines 27-28: The woman is singing and working at the same time. The speaker reminds us once again that the woman is both a peasant-farmer and poet (symbolized by her singing).