Ducks will need their own pen.They can be ranged together but need separate sleeping quarters.Ducks will yuck up the water on their first visit.Its kinda gross for the chickens to have to drink after the ducks.The other thing is the feed.The ducks can bring water in their bills to the feed.If you are using a feeder they will quickly sour and entire feeder.The other thing is the duck manure.Chicken manure in a way is sorta solid,duck manure is anything but.A chicken will scratch and turn their own bedding to a degree,a duck will just cover it...from wall to wall.And for me I raised Muscovy's who would perch and poop on the chickens.Sorry, I know Muscovy are hated here in Florida but they were loved and cherished in Michigan and I bred some really nice colors.Your biggest cost with chickens is the wasted food,your biggest cost with ducks is their bedding.It only takes a few ducks to completely cover an 8x8 in just days.On the bright side they dont waste much food.They dont do well with feeders,its better to serve them in a bowl.