What would kill our chickens?

We have found 3 dead chickens this morning. All in front of the door when we opened up the chicken house all with wet necks. Fourth one in 3 weeks. These are older chickens but they are still not full grown. Only a couple months old. We have some younger. But we just got all new chickens because over the Christmas holidays we lost all of ours to a bobcat. So now all the holes and crevices have been sealed because of snakes. The back ramp has been closed off and we shut them in everynight at 9pm when they go to bed. We check through everything before we close them in. And yet this morning we found 3 dead chickens. Any ideas?