I have a book on chickens and this is what it says on toxic foods: DO NOT FEED CHICKENS:
Jimsonweed (thorn apple)
Water hemlock (cowbane)
Yew * I've read in a few places that it's okay to feed chickens potatoes if you don't feed them green eyes or the skins. The green eyes/skins contain some sort of toxin. I'm not positively sure of this, so just to be safe, don't feed your chickens potatoes either.