What beefy said. Nine times out of ten, at *least*, livestock will not mess with toxic plants unless they've run out of other things to eat or are bored silly. (Every now and then, an animal will however get a weird gourmet whim and decide that some strong- and nasty-tasting toxic plant is yummy, and get poisoned by it even though other food was perfectly well available. Rare, though)
The main thing to avoid is anything that will kill an animal dead with just a single meal. I am not positive what this list would consist of, exactly, for chickens. But for large stock you're talking things like yew and oleander.
That said, I would not expect there to be hardly anyone who could raise their hand and say 'my chicken was killed by eating X poisonous plant', because it would be extremely unlikely you'd ever know that was what killed the chicken. Doesn't mean chickens don't get killed by eating the wrong plants, just that it can be hard to tell apart from the long list of other things that can cause the main symptom 'death' in chickens.
Some of the things on those toxic plants lists seem rather irrelevant to chickens, though, if you ask me. Like for instance wisteria... as far as I know the only seriously toxic part is the seeds, which if your chickens are reduced to wrestling their way into hard thick wisteria seedpods and choking down the large hard seeds, man oh man you need to be giving your chickens MUCH more to eat or to do with their time
JMHO,
Pat