Yes, during the first two weeks, but then one morning the males have just doubled their size over night, broke out and have pooped all over the place… At four weeks, the legs and the bills are a dead giveaway for drakes.
Maybe I'm just used to the Muscovy, they are slow!
You are fortunate. My two little Mallard girls that I got from Metzer about 4 years ago let the boys pick on them quite a bit. They are the reason I have had to rehome drakes before. Sometimes they will chase the little Call drakes but other drakes were no match for them. I am glad yours can stick up for themselves. Mine can fly also, I no longer have to clip their wings because they know where home is.
Their slow growth is often listed as one of the reasons for not keeping Muscovy Ducks.
Your Mallard Ducks are domesticated and very likely no longer have the instinct to "fly south" to over-winter. "My" mallards are not mine, those are wild birds, just coming to visit and enjoy the safety in numbers. I have observed huge differences in behaviour between my domesticated ducks and the wild Mallards. And my ducks have learned a lot from their wild cousins: How to hide in plain sight. How to drill around an autumn olive shrub to make it topple, so that you can reach the berries. How use the wings to run faster. - And that it is important to always have somebody to keep an eye on the sky.
My very first two Mallard ducks were rescues that were only a day old from the wild. It's funny I never really could tell the difference in their behavior but I am sure it is because they were hand raised by a human and took on the domestic side of themselves instead of the wild. I have another one that I just got from our Rural King. I thought it was a Rouen because of the eye stripes but seeing the back toe missing an looking closer I think it is a Mallard for sure. Just hoping for a Hen this time.
I will say though how hardy Muscovy ducklings are. I got two that were 3 days old. Kept them in the garage in the dark except when someone would go out there with absolutely no heat. They grew just fine and one turned out to be a female so I still have her. The huge brother of hers had to go down the road and got traded for some females that were smaller. Olivia is one of my Sweetest ducks and is a big duck but a sweet one at least. So was her huge brother. He never once tried to bite me or was mean to anyone. It was summertime when I did this.
I still want Muscovies, but first i need to fix all the infrastructure here. Today the waterer in the duck-house broke. One of the support studs has rotted away because it was kept moist permanently. Fixed it with dux-tape and zip-ties, but i need to repair it thoroughly.
Don't you! 
Forget our peas! Don't! - Don't! - Don't! 
