This is what I saw out the window of my car on the way home.

At first I thought it was smoke: residue from an open air fire. A large black ball in the sky, moving across the horizon. As I got closer I could see it changing in shape, moving from a circle to scattered lines, then coming back together again as it looped out and over power lines. It was a flock of birds, hundreds of them. Pulling together and stretching apart as they moved up and down & across and back. It reminded me of those plastic globes you can buy that can shrink to the size of a grapefruit, or stretch out bigger than a basketball. Expand and contract- close and far.

As I got closer I could see the flock regrouping. It moved together as it swooped down, pulled apart, then compressed itself as they reversed direction and flew away again.

Imagine if birds flew in a swarm.
Then add twilight.