This has been a terrible season for grackles. Unfortunately, grackles are very agile and much smaller than squirrels. We have found that most counter-weight feeders are not effective against avian pests like grackles and starlings. The best ways that I know to truly keep these pests at bay is to feed straight safflower in feeders they  can land on and a good cage feeder for anything with sunflower or peanuts in it. Not all cage feeders will do the trick. The cage needs to be far enough away from the food so these birds can’t reach it with their necks stretched.