Autumn is frequently used as a symbol in literature for old age, the time before death, symbolized by winter. "To Autumn" avoids any super-obvious references to death, but we do get some subtle ones, like the oblivious bees that think the summer will last forever, or the "hook" that spares the poppy flowers from their inevitable end. As the day begins to "die" in the final section, the entire landscape contributes to the song of mourning.
The concept of death can only be applied to the natural elements in the poem by projecting human experience onto them.
The mysterious and reflexive noises made by the animals in the last four lines subvert the speaker's anxiety about the death of the day.