Hello and welcome Diana22 from still drizzly southern Scotland. How old are your ducklings? Drakes tend to have bigger feet than ducks, which is fairly obvious from a young age, then they are just bigger, and longer in the body. It's a while since I kept Muscovies but I remember that when they develop their red bits on the face, the drakes ones are bigger and more pronounced than the ducks, but I don't remember at what precise age that happens.