In German the Berner is described as a "Wachhund" - watch dog - and it is my understanding that such a dog would raise the alarm at something unusual (stranger, funny noise, etc) by barking. I would also expect such a dog to calm down when the stranger is recognised or the funny noise investigated and found to be nothing for concern.
A guard dog (such as a Rottweiler) would not necessarily sound an alarm, but they often do. They also tend to assess a situation for themselves and sort out the problem accordingly. However, the Rottie Standard requires a self assured dog and as such they will be wonderful companions that do not fly off the handle, but if push comes to shove you can rely on their protective instincts towards their family. Having said all that we have one Berner who is not only a very good watch dog, but who is also very protective towards me and I think he would come to the party if ever required. He is a dog that was hand fed and reared by me and I think that as a result he has bonded particularly closely with me. Definately nurture has triumphed over nature! Marion Brown Teversal Bernese Mountain Dogs South Africa Dogs Never Lie About Love (Jeffrey Masson)