Where you have turkeys as well, its always possible chickens could get blackhead. Just a thought, here is a quick link about it. Maybe research that a bit more. http://www.millerhatcheries.com/information/diseases/blackhead_disease.htm
Blackhead is a always possibility, but the OP didn't mention any problems with turkeys, and in my experience it's the turkeys and peafowl that usually get it if any are going to get it. Additionally, of all of the blackhead poop I've seen, and I have seen many, it's never been green like that. Not saying that it isn't blackhead, just that if it were I would expect to also see some problems with the turkeys.

-Kathy