Right, it is illegal to take a migratory bird from the wild. You are not allowed to disturb a nest or capture wild ducklings (or collect eggs) to keep or sell. The law is not intended to prevent rescue of predated babies that cannot survive on their own. If you are concerned, contact your local wildfowl authority and ask for their advice. If they tell you to raise your rescue, you are good to go. If they tell you to turn it in, then do it (and let them handle the cost of antibiotics) and get yourself a few mallard babies from a breeder. Mallards and Muscovies can coexist (sometimes a little TOO well) and they make for a very pretty flock.