These females can sure fly although they are Muscovy ducks... they escaped several times this way. The male can't fly as he's too big (and lazy, to say the least) though.
I guess most Muscovy ducks raised in farms aren't used to fly anymore as they are both selected according to how fat and large they are to produce enough meat and are also overfed but the ones which have not been selected according to how heavy they can grow and have enough free space to run/swim and fly don't get that fat and can still fly
These species exist in the wild (they are usually almost fully black) and can fly very well, I then guess there are various domestic lines including ducks which are fatter and less prone to fly (selected according to their large size) and ducks which have kept some wild characteristics and morphology. These females are close to the wild morphotype although the male is indeed pretty large.