They aren't supposed to eat the medication added to medicated feed, Amprol I think?
Amprol and Corid are both amprolium, and it's okay for ducks, as is medicated feed that has it. There is one type of medicated feed that *is* toxic to waterfowl, that's a turkey starter made by Blue Seal, the version with an HS in the label is toxic to waterfowl, ducks and dogs, it contains something to prevent blackhead (histomoniasis).
-Kathy