We did start to supplement with oyster shells. When we separated the culprit her eggs were soft. We added the oyster shell to her diet before reintroducing, then we added it to the whole flock. Yes the duck lays the egg on the floor. Not sure how to manage where she lays.
We have 2 Ducks, 3 bantam chickens, and four regular sized chickens for about 2 years. Over the last few months. One of the chickens started to break open and eat the duck egg. We have tried the following:
Added more nesting material
Placed fake eggs in coop
Getting up early to get egg first...
