The ‘diminished thing’ signifies everything is past its prime. The flowers are gone, the highway dust is on everything, the season is ending, and your life is diminishing as well.   The birds’ song signifies the poet who has to write about a world, which is destroyed and alienated. Frost looks at the natural world in a modernist period in history.  It was a time when people lost faith in society. The only hope rest in the power of the human imagination, so Frost had to find the beauty under all his highway dust.