I have at least one poisonous plant (oleander) in my yard and have seen the chickens sample it and spit it out. I have also stopped worrying about poisonous plants but that's not to say there's no risk.
I have many plants in my yard that have survived the chickens. I found most by looking for "deer resistant" plants at the nursery, and others by just giving them a shot. I have found that younger chickens (<6-8 months) are more destructive as they will sample a plant over and over before deciding it tastes bad, and by that time the plant is gone. When over 1 year old they know what they like and leave other stuff alone.
To prevent digging, I surround the plant with stones or bricks. I think putting down chicken wire, jute / burlap, or other objects could help, too. This has worked very well for me.
If I plant especially important / expensive plants or fruit trees, I have used the cheap, 3' plastic netting (green in color so not too ugly) to make little protective cyclinders about 1-3' in diameter around the plants and stake them down. I'm hoping to remove them once the plants are more established, as I don't think they'll be too tasty to the chickens. I've done this around some artichokes, and some baby fruit trees in bloom as I don't want them to eat the flowers/blooms.
good luck,
Colleen