Years ago, I saw some of those birds pounce on an unsuspecting kid's french fries at DisneyWorld. Grackles lurk around all of the outdoor dining areas, and the "Please don't feed the birds" signs are useless - the birds are quite willing to help themselves to what's on your plate.
I once made the mistake of feeding the seagulls at St. Pete, Fl. They attacked, grabbing my bag of saltines away from me, leaving me running and screaming!