Admittedly I've been irregular about buying "cage-free" or other similar eggs...but the more I think about it, I will need to change my buying habits.

That being said, I had a pet duck (inside pet, in college) for 6 years.  She laid over 300 eggs in her lifetime and they made some really great custards!  I'd give her oyster shells so her egg shells would be strong.  In the early days, one of them came out like a water balloon...never saw anything like it!  Never knew the yolks on duck eggs were so large!