I have been wondering which plants might be poisonous for our chickens. Some searches on line of lists of supposed poisonous plants left me thinking my chickens will be dead soon as just about every other weed around here is on the list. I found one discussion thread on the topic and it was helpful. Below are a couple of quotes from the thread as well as the website address. These two quotes sum up the best advice in my opinion. I'm interested in hearing if anyone in our area knows it there are plants that will kill a chicken if she eats only a little bit.
i have toxic plants all over my yard and the chickens freerange all the time. as a general rule, livestock only eat toxic plants when there is nothing else to eat or if they are young and naive and eat it by accident.
I agree as others have posted, the chickens will usually leave the poisonous plants alone if they are free-ranging and have other options. If you have these toxic plants in their runs, where they have turned every other green thing into dirt, then they will start on the toxic plants. I would never plant a known toxic plant in a chicken run. I believe you would be tempting fate, they are only chickens and they will eat what is available.
(sorry I'm not sure how to make a link here)