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Chickens will eat anything green in their run eventually, no matter how bad it tastes. So I would definitely not have any potentially toxic plant inside the run. Outside the run, for free ranging, they will tend not to eat, or eat much of, things that are bad for them. But you still have to keep an eye on things, and if you notice your chickens chowing down on something toxic, fence it off. Usually if you put a wire edging around 3 feet high closely around the plants you want to protect, that can work if there's no open space that the chickens can jump down into.
Many plants are toxic, but few are actually so toxic as to be dangerous. A lot of the toxic plant pages on the internet don't really make that distinction, or make it clear what part of the plant can be the problem and to what kind of critter. The other thing is that many of the online resources only use the common and not the scientific plant name, which can be confusing.
Here are some good references on the subject:
http://www.ansci.cornell.edu/plants/other.html
http://www.vet.purdue.edu/toxic/cover1.htm
For example, we have English ivy in our backyard, which is on many toxic plant lists. I was worried about having to try to get rid of it, because that's an awful job. A very experienced poultry keeping friend of ours, who is also a landscape architect, advised me not to worry about it. He said the chickens wouldn't bother with the ivy as long as it was outside of the run. He was right! They scratch in and under the ivy, but don't eat it.