Muscovies are domestic so those ducks are feral. Like feral cats. So you don't have to worry about any laws or anything. There's only one area in the US where they're native, some place in Texas I think. That's the only place they're protected.
If you see any that aren't being cared for or are injured/ill looking then I'd take them if I were you. It sounds like the stray ones were either abandoned or something happened to mom.
They'll all mostly die from predators etc, even the ones with a mom, so any ones you take will be lucky. I deal with this all the time because I have a half acre pond in my yard and feral muscovies and mallards basically adopted me. They have a lot of ducklings every year (some scovies hatch as many as 26 in one brood! they're excellent brooders) and some abandon their ducklings, some ducklings need help because of deformities or getting hurt by a predator etc.
This is false. Since muscovies are native to ANY part of the US, they fall under the federal migratory birds act.
If they are feral, you need to contact the wildlife department before interfering, IMO.
