I've never had one do it, but I had one tithonia that had fallen over and had a bloom laying close to the ground. It was there this morning, and now looks like something either bit or pinched it off. My money is on the neighbor, who hates anything that goes even 1/2 inch out onto the sidewalk, but could be a squirrel, I guess. Anyway, I staked it up onto the hibiscus plant, so we'll see what happens.