Howard Nemerov (1920-1991) is a wonderful poet of autumn. (He is also particularly good when it comes to trees and snow.) Here is a poem of his for this time of year.
September, and the butterflies are drifting
Across the sky again, the monarchs in
Their myriads, delicate lenses for the light
To fall through and be mandarin-transformed.
I guess they are flying southward, or anyhow
That seems to be the average of their drift,
Though what you mostly see is a random light
Meandering, a Brownian movement to the wind,
Which is one of Nature's ways of getting it done,
Whatever it may be, the rise of hills
And settling of seas, the fall of leaf
Across the shoulder of the northern world,
The snowflakes one by one that silt the field . . .
All that's preparing now behind the scene,
As the ecliptic and equator cross,
Through which the light butterflies are flying.
Gnomes and Occasions (1973).