Being a HUGE dog lover, I would not shoot it but figure a way to keep my flock safe. It is a natural instinct of the dog (a dog we had killed a neighbor's cat, our dog never went in the front yard again. We were so mortified by that). If it is a stray, animal control should be able to catch it and hopefully adopt it out or find the irresponsible owner. Once fair warning is made, then, what happens happens.