I have kept chickens, and have kept call ducks, but we did not keep them together, each was in its own pen. The chickens were also free range - loose, but the ducks had duckings and we knew the cats would find that too tempting, so the ducks were kept in a large pen where the cats could not get in.
I often am asked if you can keep ducks and chickens together. Many people want to do this year round, but more often it is a consideration for the winter as it is easier to have all outdoor animals together rather than going from pen to pen through deep snow.
One of the main considerations in regards to keeping ducks and chickens together is food. Ducks cannot have medicated chick starter. The medication is toxic to ducks, so if you are feeding medicated chick starter or any medicated chicken feed, you cannot have ducks in the same enclosure.
Another concern is that ducks need water in order to eat their food. They will often take a mouthful of food and dunk it in the water to soak it. They do this because they do not have saliva. This can make the water very messy for chickens to drink from.
Of course the other concern is that while ducks love to swim chickens cannot swim at all and will drown. Even in a shallow pool chicken feathers can become waterlogged and will pull the chicken down if it cannot get out. Ducks do not really "need" water for swimming, and in the winter when it is freezing cold they should not have water for swimming anyhow.
Aggressive roosters can be a concern too, sometimes aggressive drakes can be mean. Always watch animals when putting them together. Make sure you provide enough space so animals can get away from each other if they are being picked on.
Personally I would not recommend keeping ducks and chickens together in the same pen, as they are better off separate.
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