Are you sure they are Muscovy ducks? Where do you live? Muscovy's are usually domestic. How old are the ducklings (or how big)? If they are still tiny and easy to catch, you can raise them. Just search for duckling care on the internet. If they are hard to catch and older, they are probably better off trying to fend for themselves. If you can't care for them (or they are in fact a wild species making their possession illegal), take them to a wildlife rehabilitator. My wildlife page at
http://www.fishpondinfo.com/wild.htm has a link to a list of rehabilitators in the US if you're in the US.