The only problem I've had was that my drake kept mating with the hens (big hefty meatbirds). I think he actually mated a couple of them to death. The bigger ones seemed to handle it okay, but they definately didn't like it, and it was difficult to listen to, if he got started I had to break them up. I have rather small coops, so I won't put ducks with my egg birds, they're smaller breeds and I'd like to keep them alive.
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Cory