adding ducks to a chicken flock
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Each flock makes up its own mind, so there is no way to predict exactly.
Generally speaking, ducks like to be with other ducks (there are exceptions)
Drakes can try to mate with chickens and kill them due to equipment size mismatch
Ducks have a different relationship with water than chickens do, and chickens find it appalling.
I realized you aren't a mindreader. Sorry.
Ducks love to splash, they love to put things in their water, they grow ripsnorting fast, much faster than chicks, their bills cannot handle food the same way chickens' beaks do. Ducklings need three times the niacin chicks do (some ducklings have a harder time with this than others). Ducklings need to be able to wash their little heads frequently to prevent infections. They don't roost.
Many people have both ducks and chickens. Some house them together. I hope some duck-chicken folks check in here. I am just a duck person.
Edited by Amiga - 1/6/13 at 3:00pm
I had four chickens (all hens) when I introduced three Indian Runner ducks (also hens).
The chickens certainly rule the roost, and despite the fact that the ducks are much larger, they still defer to the chickens in matters of first crack at the kitchen scraps and feeding time.
However, aside from a few warnings from the dominant chicken hen, they all coexist nicely. In fact, the ducks now pester the "runt" of our chicken flock.
Ducks require their own water because they are super-messy. They love to put dirt and mud into the water, it's like an obsession. I have no idea why they like to do that, but they just do, and if you can find a way to ensure that the chickens have their own non-muddy water, that would be key. The ducks also need a little pool/pond to splash around and clean themselves in.
Otherwise, everybody seems to be getting along well. Ducks are, in my opinion, a much bigger commitment of time and energy (lots and lots of water changing). But they are absolutely hilarious, and some breeds are fantastic egg producers. Duck eggs are pretty much all we eat these days, and our girls are laying just about an egg a day, every day, even during winter.