Hi Kristi, welcome! Try installing a 3/4 wooden predator guard over the entrance hole. Then in the Spring and Summer after the bluebirds build their nest make sure the nest is not built up to the entrance hole. Remove an inch or two of the nesting material from the bottom of the nest. Hopefully this will make it difficult for the grackles to lean in the hole and snatch a baby. Hope this helps!
Dana,
Lancaster, PA