I have never had young ducks interested in oyster shells before, but I've just moved out my 7 and 9 week old ducklings into the big coop and for the last several days they've been eating them. They have grit and have plenty of time on grass, but they seem to really like the oyster shells. I'm so confused - I went to the trouble of ordering an all-clock feed that has lower calcium because I thought the higher calcium in my layer feed was bad for the ducklings.