The thing about those toxic plant lists is that context is everything. English ivy is poisonous, but it also tastes bad. It's very unlikely that a chicken free ranging in the yard is going to eat enough of yucky tasting English ivy to suffer toxic results. However, a bored chicken kept in a pen, with nothing to do and nothing else green to eat, just might eat enough to harm it.
I agree that we should be careful about what plants we intentionally give to our chickens, but I think we can relax a bit about what is growing out in our yards.
Similarly, if you're seeing some of your chicks dying, of course it makes sense to rethink what you're feeding, cut back to only chick starter/fresh water, and see if that helps. But if your chickens are thriving, it probably doesn't make sense to stop what you're doing.
