I have a duck that has been laying shell-less eggs. I mix oyster shells in with their feed, but I still get these weird eggs! She laid one every day for about a week, then nothing for about two weeks. Now she's laying them again, and again they have no shell! I also noticed that she lays them late in the day. All of my other ducks lay their eggs early. I collect them around 8 am and everyone has laid one by then. But when I go out again around 6 pm, I have the egg with no shell. Anyone else have this problem? Or know what's causing it? I know a calcium deficiency will cause this, but like I said, they get oyster shells on a daily basis because it is mixed in with their grain.