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

We let out the hens to roam during the late afternoon but we but them back in there coup before nightfall. Then a couple of days later some had died. We have 3 hens and a Ginny left they are about 3 to 4 months old. Can anyone give me a list of pisionous plants that the might have eaten or have any other ideas on what is happening. We live in Rusk in East Texas. Thanx.