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I guess that depends on the approach and what you are comfortable with. I think it was Joel Salatin who had rabbits in mesh bottom cages over the chooks, so any poops and food dropped through to be dug over or eaten by the chooks.

Being in the run itself, I think some make it successful, but if you can manage to have them both getting along (usually the rabbits terrorise the chooks or the chooks peck the rabbits), then you need to be cautious on the foods that one can eat, but the other can't. So nothing long and fibrous that could give the chooks an impacted crop and nothing fermented that could make the rabbits sick.

Thanks. I'll probably keep them separate but have a small (bunny sized) entrance to the chook run so that the rabbit can go in there for a bit more space.

I have a friend in the Philippines who keeps goats on the top level, chickens on the next, then fish on the bottom. It seems to work well. I'd try and work a worm bed in there somewhere too.