Some thing taking our polish hen's heads. Can someone give me ideas. . . ?

Yesterday morning I went out to feed the chickens and I noticed that there was a dead one, well its head was gone and it was one of our polish hen's black and white. Then this morning I look out the window and see another one dead, go out look and its a headless polish hen black and white. We live in AZ and we don't have foxes. Our chicken pen has a fence around it and they have a coop that they sleep it. It has huge trees so its hard for a owl swoop in, I think it is a person but i wanted to ask you all if you had any ideas and see if you all think its a animal. Also what can I do to keep things out?