Ok I free range on property that we built our house on 2 years ago that was wild native. I have not ID most plants on it. I do know we have posions ones for some of those i have ID. Like horse nettle, posion oak or ivy, wild privet ect. Chickens eat whatever they want and I have not lost one in a whole year to feed. Hawks yes forage no. Thing is they might eat the poison plants but because they eat so much other stuff it will be minor and not effect them. I have seen my chickens eating paint that was on the grass or off their coop. It never hurt them and coop was fresh painted month before. So chickens can eat thing that we would not and can get away with it as long as they are eating a varried diet. I would not put them in an enclosed pen with just poison plants in it and expect them to survive for that would be overdose. But eating some here and there will not hurt them.