Late last afternoon (September 20), I went for a walk at my local natural area (where this image was made). Soaring over the fields--and just over these fields, not over any of the other 100 acres of grasses and wildflowers in the preserve--were at least 50 Chimney Swifts (Chaetura pelagica) .
They were soaring, diving and hawking at all levels, from hundreds of feet in the sky, where they appeared as tiny dots, to just above the tops of the grasses. I've never seen such a swift feeding frenzy before; it was amazing.
I know where the swifts are gathering each evening in preparation for their southward migration: an old brick chimney associated with the heating plant of a college about a mile away. Watching the birds disappear into the chimney each evening at dusk is a fantastic spectacle, too.