I would like to make some recommendations that may help you deter grackles coming to your feeders.  First, do not feed anything such as bread, table scraps or leftovers. Grackles will take advantage of any food source.  Also, don't allow bird seed to accumulate under the feeders.  I would suggest you use a high quality bird seed that does not have corn or milo as filler.  These seeds are a grackle magnet and not well liked by many of the species you want to attract.