Gosh, that is awful! Hope your flock will have a speedy recovery and kudos to your pregnant wife!
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Some of you may want to go back to the beginning and read the OP's post again. I believe "the mutt" has already been dealt with.....harshly. (and appropriately)
And if birds are allowed to range about in the open, such an experience should not be unexpected. It is the nature of dogs. Not all of them, of course, but it only takes one to create such carnage. So it is incumbent upon us to get proactive to protect them. We might wish that were not so, that we should not have to worry about someone else's dog, but the fact is we do. Or as someone once said, stack all your droppings in one hand and your wishes in the other and see which one piles up first.
And as for dogs (and many other predators, but specifically dogs), the best pro-active defense you can have is an electric fence. Mine has whapped so many dogs around here, and sent them packing, I've lost track.
https://www.backyardchickens.com/threads/dogs-and-electric-fences.1210854/
Electric fences. On guard 24/7 and always at the ready to mete out a most severe punishment. Not lethal, but to a dog, unforgettable. Not many ever return for a 2nd dose. If you are going to let your chickens roam about, don't try it without an electric fence.