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I've got a much better solution.

Hire a bunch of teenagers, "issue" them "Airsoft" guns (preferably battery-powered, fully automatic), and have them go to town on the birds whenever and whereever they find them between 4:30 PM and 7:00 PM. Just have them hose down every roosting tree with those little plastic pellets.

Grackles ain't stupid. They'll clear out within a week. Have the kids keep patrolling for another week or two, and the birds will likely not return for quite a while.

Another solution would be to allow the kids to shoot them with steel BB shot, but knowing kids, you'd likely end up with a BB gun war at some point. You'll still have that with Airsoft, but the pellets don't hit nearly as hard. They'll sting, but not break the skin.