You might want to post this on the bird forum or in the wildlife forum. I've seen many such questions in both of those areas.

Hi jdh!

Grackles are protected by Migrotory Bird Act, so be careful of what you do. Many birds destroy other nests when found in their territory like the Grackle. The most pugnacious is the House Wren. They go from nest to nest piercing eggs with their sharp beaks.

Their way of protecting food source for their young.