The author's repetitive use of the "b" sound in the excerpt creates a sense of rhythm and musicality to the lines. This repetition also emphasizes the image of the birches bending, as the repeated "bend" sounds mimic the visual movement. Additionally, the repetition of the "b" sound suggests a soft and gentle motion, reflecting the peaceful and natural imagery of the birches swaying in the wind. This use of sound devices enhances the reader's experience and reinforces the overall theme of harmony with nature.