I use oyster shells and put them in a separate bowl. That way the laying ducks can take what they need and the drake and the younger girls don't get more than they need.
sorry to sound like a novice and go a bit off topic, but what does a oyster shell do and why do they need it? Ive only had my ducks for a week now so im just getting to grips with things
oyster shell provides extra calcuim to help with creating a hard shell on the eggs. Sometimes if they don't have enough calcium the eggs will come out soft or with no shell at all.
Oystershell and grit serve 2 different purposes.
Grit is to grind their food in their crop before it travels to the stomach.
Oystershell is to provide calcium to layers.