I feel for you, I know that once they (dogs) have learned a behavior like killing, it can be extremely difficult to stop, you can't ever fully trust them, you may have to tie your dog, or kennel it when you are gone, and until you can solve the issue. I have a few puppies right now, and chickens everywhere, mostly they are bored with them, but have shown some herding (stalking) instinct in the large field...I don't allow it to continue because they are for predators, not herding chickens! and I don't want them getting ideas, I don't really know your dogs and lotta people have offered good advice, just wishing you good luck with your problem.