Tried to delete my reply when I saw Q already answered below but wasn't able to!

The slugs chew on the leaves, but the squirrels can gnaw through the woody stem; rabbits too.

I 've found snails in the flower heads so yes slugs/snails likely culprits. We find snails under the eaves of the house so climbing not a problem.

Snails can't remove the entire seedhead and carry it away. That's a squirrel.

Didn't say the seedhead had been entirely removed and carried away though?

It's a squirrel. Snails eat leaves, not seeds. They do same to my sunflowers.