Chickens sick

I am new at this but ill give it a try because I really need your help.I have 9 backyard chickens 5 roosster 4hens ,2 are 8months old and 7that are 6months old.I also had 9 plymoth barred rocks,5 roosters,4 hens.I recently lost 4of the rocks 3roosters,1 .hen.I also have one of my backyard roosters sick.I noticed my chickens just sitting there on the ground with their eyes closed like they were sleeping.their heads were kinda twitching and their tails were hung low ,their feathers were ruffled and they went like this for a few hours.Then they would open their mouths like they were panting and then they would die.My one rooster is eatimg and drinking alittle ,but he hangs his tail lkow and he has ruffled feathers.I had to remove him from the coop because the others were beating him up or they would have killed him.Now im new at chickens could they have been poisoned or is this an illness,?