Final answer:
The main theme of the poem is that some things endure longer than others.
Step-by-step explanation:
The main theme of the poem is that some things endure longer than others. The poem describes how time affects different aspects of the natural world, such as the movement of flocks, the changing of seasons, and the fading of the song of the nightingale. These examples illustrate the idea that some things, like rocks and the natural world, have a lasting presence while other things, like the song of the nightingale, are temporary.