I know the biggest difference is in size. Rouens were bred to be meat ducks. Mallards are thinner, smaller. Rouens cannot fly...or not much. Mallards fly and travel. My question is this, is there a difference in markings, coloring or anything else that can be used as a reliable means of telling the difference? I ask because my little female that I bought (from a feed store/hatchery) to be a companion to the baby, was bought from a flock of Rouens all about the same age. She was the smallest of the bunch (around 30-40 birds in that pen) and thought to be a runt or simply small. Thats what I wanted for the duckling since there were no hatcherys shipping at the time because of the excessive summer heat...too many losses en route. I needed a small juvenile female that would not attack the baby...Chicky baby was perfect for the job ( and has done exactly what I hoped for ). So this little girl (Chicky) is supposed to be about 3-4 months old. She was almost feathered when I got her but it was hard to tell because she was a little tattered and ragged from being beat up since she was so small compared to the other Rouens.....She did heal up and feather out nicely and evenly after a few weeks. She has always been the most trusting and gentle out of everybody. She allowed me to hold her the first day. That was a surprize too... I've seen her take a short hop out of the pen, and I actually count on her to hop out of the bath tub when shes been playing with the baby....if she sits on my shouder she will flap to balance herself and stay up there as I walk. BUT, last night that little 'Rouen' got impatient waiting on me to chop some greens for their dinner and she FLEW all around the living room, full spread, tail spread wide, circled the room then flew into the kitchen and made a graceful landing on the counter next to where I was standing.... ! I was MORE than surprized. Shocked actually. I had no idea. It was like a different bird. So, between the size and the ability thats shes demonstrated I am wondering if she is more Mallard than Rouen and how to tell? I've never seen a true Rouen fly like that. My 2 adult Rouens can fly but its a heck of an effort and they can;t do it for long. They CAN fly straight up,. but they can;t control their flight as well as a bird that flies as their normal routine. They struggle and its obvious because of their size. They're like bowling balls with wings...big heavy and well rounded. Buddy is about the size of a butterball turkey (feathers make a lot of that) and his mate Squeek is close... But this little girl is only 2/3 their size, maybe only 1/2. So thats got me wondering.... How can you tell the difference between a female Rouen and a Mallard? Its curiousity more than anything.