If you can get waterfowl starter, that's best. I could only get turkey-waterfowl, and the protein % was higher than recommended in Storey's Guide, so, following SG, I added one part oats to four parts crumbles (so glad I was awake during algebra class).
If all you can get is chick starter, then you probably want to add either a sprinkle of brewer's yeast on top of their food, or 100 mg of niacin per gallon of water, since ducklings need three times the niacin chicks do, and people who don't realize that often have duckling leg or nervous system problems till they can supplement the niacin.
I gave my ducklings a little gro-gel their first week, also, and keep poultry vitamins-electrolytes-probiotics mix handy for times of stress or illness or injury.