You can feed ducklings Chick Starter. Make sure you give them plenty of Niacin as they need 3 times as much as chicks. It's water soluable and you can pick it up at walmart for about $10 a bottle (it'll last you FOREVER). Ducklings need 10 mg of niacin per day. It's best to put 500 MG into 8 gallons of water and give them that, most of it will wind up on the ground but they should be able to get the amount they need. They will not Over Does on vitamins. This is important. If they don't have the amount of niacin they need they will have poor leg development and poor brain development. They will die.
If you feed your ducklings chick starter, make sure it's NON-medicated.

