I had one of those huge flocks of starlings and grackles swirling around the treetops in perfect unison. They flew above the tree I was in and the flock must have made an instant turn. The air forced down from those thousands of wings made the entire tree shutter and shake like there was a minor earthquake...no exaggeration! At 1st, I really didn't understand what happened, but it happened again a few seconds later and I was in a fairly large tree (20"DBH)...it was not some little sapling.
I'm still amazed when I think of that encounter.