I would try to have some kind of calcium--eggshell or oyster shell--available at all times. You can toss empty eggshells on the floor of the pen (step on them so they are no longer egg-shaped); you can crush eggshells and put them in a dish; you can put oyster shell in a dish; and of course you can do multiple things at once if you wish. My experience is with chickens--I typically toss empty, crushed shells on the floor of the pen, and make sure there's also a dish of oyster shell just for good measure. I think the same would work with ducks.
I'm guessing each female needs to eat about 2-3 eggshells to make a single new egg, but that is purely a guess. As long as they always have some available, they should be able to regulate the amount for themselves.
