Naturally, some plants are poisonous to chickens, just as there are plants that are poisonous to other pets like cats and dogs. It's problematic to come up with an absolute list of what not to have in your yard, though. In many, if not most, cases, just having a plant in your yard that is poisonous won't necessarily cause problems.
For instance, daffodils are poisonous to most animals, including chickens. But many people have dogs or cats as well as pretty spring gardens in their yards, because dogs and cats generally won't eat these poisonous plants. Similarly, my chickens just aren't interested in eating my daffodils.
Remember, chickens are foraging birds by nature. They will take a small taste of most plants to determine what is good to eat and what is not, and they seem to have a pretty good ability to determine what to stay away from. If you allow your hens to raise your new baby chicks, the hens will teach the chicks what is good and what is bad.
Whether your chickens will eat things that are bad for them will also depend on your particular set up. For instance, if your birds have a small, bare run with nothing else around to forage and eat, they may finally get tempted enough to consume what they really wouldn't have touched otherwise. It's the same with dogs and cats. While your cat or dog isn't especially likely to eating your foxglove outside, house plants can be a temptation, because they are the only plants around. So, if you give your chickens plenty of access to fresh pasture with lots of good things to eat, they are less likely to bother the plants that will hurt them.
Last, if YOU offer your chickens something to eat, they are likely to snap it up without taking the time to evaluate it themselves. Since you are the "Almighty One Who Provides Food and Treats," your birds may simply think that everything you offer them is good, and they may eat what you give them even when they would never have sought it out themselves. So, be sure that when you feed your birds table scraps, the left overs are not only good for them but also in good condition. In other words, don't clean out your fridge and feed them old or moldy food, thinking that the birds won't eat it if it's bad. If you are giving it to them, the may eat it just on trust alone.
An incomplete list of plants that are poisonous to chickens includes daffodils, foxglove, morning glory, yew, jimson weed, tulips, lily of the valley, azaleas, rhododendron, mountain laurel, monkshood, amaryllis, castor bean, trumpet vine, nightshade, nicotiana, and tansy. Again, that list is incomplete, and you may be able to keep many of these plants in your yard, despite the fact that they're poisonous. If you want to be extra careful, though, whenever you add a plant to an area that your chickens access, you should use Google to find detailed information about it, so you can see if there are any potential problems for your chickens and other pets.