The speaker uses alliteration in small moments to enhance the sound of the poem's language. For example, take the first line, when the speaker alliterates the loud /b/ sound:
When I see birches bend to left and right
This moment of alliteration creates a strong but somewhat staccato or choppy rhythm. In turn, the alliteration accentuates the poem's meter, since both of the prominent /b/ sounds land on stressed syllables, thereby strengthening and establishing the iambic rhythm (da-DUM da-DUM) that will run through the rest of the poem. It also simply draws attention to the poem's most important image: that of the bent birch trees, weighed down by the leftovers of an ice storm.
The alliterative /b/ sound repeats in lines 3 and 4, where the speaker also uses an alliterative /d/ sound in line 4:
I like to think some boy's been swinging them.
But swinging doesn't bend them down to stay
These voiced consonants add texture and heaviness to the poem in this moment, subtly evoking the weighty bending of the birches themselves.
In other moments, alliteration adds a sense of lightness to the poem's lines. This is the case in lines 24 and 25, when the speaker layers the gentle /f/ and quick /t/ sounds:
As he went out and in to fetch the cows–
Some boy too far from town to learn baseball
This lively alliteration reflects the vivacity and curiosity of the boy being described. It also creates a varied and fresh sound, which keeps the speaker's language from sounding dull and predictable—something that is especially important in a poem like "Birches," which is nearly 60 lines long!
In another striking example of alliteration, the speaker turns to the hard /c/ sound to again evoke the imagery at hand:
As the breeze rises, and turn many-colored
As the stir cracks and crazes their enamel
Here, the harsh alliteration mimics the "crack[ing]" sound the ice-covered trees make as they, ever so slowly, begin melt in the sun. The /cr/ sound of "cracks and crazes" then echoes throughout the following lines with "crystal" and "crust," subtly imbuing the poem with the crackling sound of the trees.