I am going to disagree with the others. Medicated chick feed USED to have stronger drugs in it which were harmful to ducklings, which eat much more than chicks and therefore overdosed on those drugs. No longer. Now there is only the amprolium to increase resistance to cocci. I have raised several broods of ducks with no problem whatsoever. Amprolium is SAFE for WATERFOWL.
The old hype is being propagated all over the InterWeb. People often repeat it simply because someone at a feed store or (on a discussion forum) told them, "Oh no, never feed medicated chick starter to ducklings - you'll kill them!"
Chick starter, medicated or not, does not contain enough niacin for growing ducklings. I simple add powdered Brewer's Yeast to their feed a couple of times a week.
Again, no problems with those I've raised. And I'm not the Lone Ranger in this regard; there are many, many folks on BYC who know it's safe to feed medicated chick feed to ducklings and have done so without causing any harm.