Our chickens are dieting and we can not figure out why!?

We let our hens and Ginny roam around during the late afternoon wil we watch them. Then bring then back in the coup before nightfall. Then a couple of days later 2 or 3 hens died. 2 of the hens and the Ginny are about 3 month old and one hen is about 4 months. If anyone can give me a list of poisonous plant they might have eaten or have any ideas of why or how this is happening. Thanx.