You are better off to fence a separate area for you dog or chickens to keep them apart. A muzzle could cause your dog to overheat, and it does not sound like either of you are planning a good life from this point forward. If you can't physically keep her away from them without all kinds of stress to you both, rehome her and do everyone a favor. It is not fun to keep a dog you no longer enjoy, IMHO. Good luck though, this is a tough call.