Both chick starter and scratch grains are inadequate nutritionally for ducks. They require a balanced feed with vitamins and additional niacin, 2.5lbs brewers yeast per 50lbs of feed, they need more like 3.5lbs when they start laying , or 100-150mg niacin per gallon water, or 35mg niacin from 0-2wks and 30mg from 2-10wks, more isn't better. The niacin (brewers yeast is a source) is essential for them to absorb other nutrients, ducks should have access to pasture as bugs are a primary source of natural niacin and protein. Rickets is often confused with niacin deficiency and is caused by vitamin D deficiency or phosphorous/and or calcium deficiency/imbalance. Make sure the feed isn't too high in protein, it should be around 15% from wk 2 pretty much on (you can give more protein to layers and obviously meat birds are different), you can bring the protein level down a bit by adding oats. If they don't get enough excessive and eat too much (especially without niacin) they easily develop shaky legs and become lame. Ducks are "allergic" to medications and preservatives in feed, make sure in is medication free. We give a blend of probiotics to all our ducklings and then occasionally to our ducks, probiotics are especially helpfull with sick ducks and will help them recouperate after antibiotics, you can use live cultured yogurt for this. Hope this is helpful.