Grackles, or starlings? How big of a bird, are we talking about, in this event?
We'd get flocks of 10s of thousands of starlings, when I lived in KY in the 70s. Our street was lined with poplars, and they'd roost and make all kinds of noise, and one heckuva mess. Really obnoxious.
"Grackles, or starlings?"
Could be similiar birds. Basically a pest.
Last fall I was in downtown seattle on 3rd avenue where there were literally thousands of small birds congregating for a few weeks.
One day I heard a strange squauk from the tree above me. What I saw was a Falcon sitting on a branch working it's claws into the branch. On closer inspection I realized it was crushing one of the smaller birds that was protesting in strange squaks. After a while, the smaller bird stopped protesting and the falcon flew away. It was actually quite amazing.