Some plants are poisonous to chickens although fortunately they are quite savvy about what not to eat and it is very rare for a chicken to eat poisonous plants due to their bitter taste.
Prevention is far easier than cure. If in doubt, keep potentially harmful plants away from your birds.
Here is a list of some plants that are poisonous to chickens, it is by no means exhaustive and I haven't been able to verify whether they really are poisonous of course (I'm not going to test them on my chickens!). The list is based on what veterinarians have told me and I have picked out of old poultry books. If you have any plants to add
Blue Green Algae found in water is also fatal so ensure water sources (e.g. ponds or water containers) are clean.
If you know of any other plants that are poisonous to chickens then please let me know - ideally with a reference so I can look it up before adding it to this list. Thanks.