If your duck is laying soft shells they have a calcium deficiency. The best example is to think of a deficiency is that your duck has a "tank" to hold calcium, 0 being empty and 10 being full. With the deficiency your duck is running on 0 and the Layer feed will fill the "tank" to let's say 5 which is enough to form an egg, but the "tank" is still only at 5 so you get a soft shelled egg not a hard shelled egg like you would with a full tank. You are just barely maintaining a half "tank" with the layer feed, but still not able to fill the "tank". Adding a calcium supplement helps boost the levels to over what your ducks tank can carry which is necessary for your ducks body to not only restore their calcium "tank" to full, but also be able to maintain that level in the future. Then after, when you feed your duck layer pellets their calcium "tank" level isn't starting at 0. Also for future maintenance offer oyster shells on the side so when the layer feed is not enough calcium to keep your ducks "tank" full they can self regulate their intake and hopefully prevent any future deficiencies.