He would declare and could himself believe
That the birds there in all the garden round
From having heard the daylong voice of Eve
Had added to their own an oversound,
Her tone of meaning but without the words.
Admittedly an eloquence so soft
Could only have had an influence on birds
If call or laughter carried it aloft.
Be that as may be, she was in their song.
Moreover, her voice upon their voices crossed
Had now persisted in the woods so long
That probably it never would be lost.
Never again would the birds' song be the same.
And to do this to brids was way she came

This is my second-favorite Frost poem, Never Again Would the Birds' Song be the same. It is a poem in the subjunctive mood, so to speak, the affirmation of an argument through a secondary speaker. The poetic speaker, that is to say, paraphrases the argument made by the "he" of the first line. It is a "just-so" story, explaining how the birds got their song. In keeping with this, it advances its case tentatively. Hence the importance of the meta-discursive markers and modal verbs:

"He would declare and could himself believe that... Admittedly... Could only have had ... If... Be that as may be... Moreover..."

The "just-so" story is actually a sort of piropo (elaborate poetic compliment) in disguise: "Your voice is so beautiful that..." The third-person narrative voice distances the poem from the "implied poet," making the emotion more poignant.

Prosodically, the poem is a marvel, those long, balanced sentences that cross over the boundaries of the 4-4-4-2 structure of the Shakesperian sonnet! Notice how lightly punctuated the poem is. The tone lies somewhere between the conversational and the elevated. The prosaic dimension of the poem arises from its argumentative structure, and serves to temper the fanciful lyrical conceit.

The reader of the poem hears the "oversound" (overtones?) of the woman's voice in the bird calls. Exquisite! Frost's own overtone series.

It's a poem that hasn't been spoiled by overexposure and ill-use, unlike "The Road Not Taken," "Stopping by Woods," or "Mending Wall." It's been anthologized, but not as much as these.