I free-range my chickens and have all kinds of poisonous plants in the yard...in fact most plants are poisonous in one way or another.
Brugmansia,
Datura, Castor Plants are the three worst ones I have. The chickens seem to know and stay away. In fact, my chickens will only eat stuff like sunflowers (scratched those right up, they were on one side of the shed and the castors on the other--castors untouched.) They don't really eat cucumber leaves or anything that is fuzzy like those. They leave tomatoes alone too. Well, the leaves at least
Datura, Brugmansia, and Castor can kill. Eating any part of the datura and brugmansia causes severe hallucinations followed by coma and then death. Eating castor beans (the seeds, from which the poison ricin is made) as few as three seeds can cause extreme vomiting, GI upset, diarrhea and eventually coma and death.
I did have a young turkey eat a purple datura flower once, it got really lethargic for a few hours and I found it dead the next morning.
The plants are beautiful, and I haven't ever had a problem with the chickens or other turkeys messing with them.