There are potentially hundreds of plants which are toxic to chickens, and normally chickens will know to avoid them, unless really hungry. It is however useful to know about some you may find in your garden, to be aware of if you have young or inexperienced birds.
The weed Ragwort is becoming more abundant with warmer summers, so look out for it turning up in your garden, its very prolific and if you see one they're be more. Always wear gloves when touching or removing ragwort, leaving any roots behind will produce more ragwort next year.
This is not an exhaustive list so if you are unsure about any particular plants you have, its worth a check on the various internet information sites for further guidance.
Teresa Benham hensforpets.co.uk