This is a very late comment, but my backyard is bordered on one fence with very mature oleanders. Every part of the plant is poisonous, including the smoke from burning the wood, but my chickens have never eaten any. (At least not enough to cause any symptoms I could observe.) They do, however, like to eat banana tree leaves. So I transplanted several banana trees into a corner that they like to congregate in for shade. They can't reach the tallest leaves. Same with elephant ears - they love to eat the lower leaves, but the two I transplanted are over 7 feet tall. I have also planted a pineapple guava in their run - it will grow huge, they don't eat the leaves (apparently) and the flowers and fruit are edible. Free chicken food! I keep them out of the yard except on rare weekends, as they like the grass too much.