Ducklings have a niacin deficiency. Commercial duck feeds have niacin in them but most places don't carry commercial duck feed. If you feed chick starter, you are supposed to add the niacin.
I have never done it either, but after reading Storey's guide I am looking into buying some. It's flippin expensive though.
I use Dumor chick starter/grower 20% for chicks and ducks from Tractor Supply and it does have Niacin in it. Niacin is vitamin B3.