Answer: Alliteration
The first line of the poem is:
"When I see birches bend to left and right"
While the third line is:
"I like to think some boy's been swinging them."
The sound device that Frost uses is alliteration. Alliteration refers to the repetition of the same initial consonant sounds within a group of words. Alliteration is commonly used in order to cause an emotional effect or to increase the persuasiveness of a text. Alliteration is also known as "head rhyme." Frost uses this device when he writes "birches bend" and "boy's been."
