Duck feed is really not that easy to find here just depends on where you live. I feed chick starter and add nutritional yeast to my duckling's food when they are young. Chick starter doesn't have enough niacin for waterfowl.
here is info on niacin deficiency...
They really need a good feed [not bird seed] especially when so young. Are you in the US? much easier to find feed etc here than many other countries. At around 3 weeks is when you may start to see problems with their legs, having problems with weakness, bowed legs beginning and then lameness...