I thought it must be a very strange wind that kept changing course.
Finally I figured out that there were several American Goldfinch feeding on the flower centers where the seeds are forming.
The birds blend in so well that it is easy to miss them until they come out from the full part of the flowers.
I usually see them feeding on the purple cone flowers (echinacea) where they aren't camouflaged as well as they are on the black eyed susan.
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