I just finished reading an article about grackles, and it jogged something in my memory. Several times over the years people have posted here complaining that something trashed all their marigolds and wondering what would want to eat them. This article mentioned that grackles often do an activity called "anting", where they let ants crawl all over them to secrete formic acid, which repels parasites. Apparently grackles also use marigolds for the same purposes, rubbing the shredded plants on their feathers.

Now we know!