I have a lab/border collie mix, and frankly, I don't trust him when it comes to being around my chickens. He'll be perfectly fine for months, then out of the blue he'll kill one or more. I know border collies have a high herding drive, and from whatching his reactions in the past, I'd say its when the chickens get a little to vocal or flighty that triggers him to "play" with them. At night he is worth his keep, the hens are quietly sleeping in their coop, and we don't have any predator problems with him patroling the yard. What I'd suggest is if your dog is very well behaved, introduce him to the chickens with some type of off command ("leave it" for mine), watch how interested he is in them when your around, then go somewhere that you can observe him without him knowing it. Make sure its somewhere you can intervine fast if things get wild! But thats just me, and its just a suggestion.