Two of the watch dogs I have lived with, Jenny the Rough Collie and Wills the Miniature Schnauzer.
Notice that neither has a huge, vicious appearance.
A watch dog does just that; they watch, they listen, they observe. Then they bark. They don't attack, they don't lunge at people, they just raise an alarm that something is different from "normal." If you live with a dog for any length of time and listen, you will often begin to hear a difference in their bark if they are announcing a strange dog is approaching, compared to a strange person. Of course, some dogs simply have an outraged bark no matter what is happening. I have a neighbor whose frustrated little dog only goes out to be tied up. She tends to bark angrily with the same tone at everything. This makes for a poor watch dog because her owners are so used to her just barking they don't bother to check if she might actually be barking at something noteworthy.
On the other hand, when a dog voices different barks for different purposes and the person learns to listen, barking dogs can be very informative. I can tell by Jenny's bark if she's seeing/hearing another dog or a person. She has an edge to her bark when another dog is approaching that she doesn't get when she sees a strange person. She also has one tone for when someone is walking by outside and another when someone steps onto the porch. Then there is the duty bark. This is when she is letting me know that someone whom we trust is approaching; she informs me while letting me know that this information is a matter of protocol rather than impending danger.
About four summers ago I was sleeping soundly when Jenny woke me with a bark I had not heard before. I was momentarily confused -- it was dark out, about 1:00 a.m. and she was clearly agitated. Sometimes in such a situation an owner might be tempted to tell the dog to be quiet and go back to bed. I think though when a dog is telling you something out of the ordinary is happening that this information is worth checking out. Sometimes a dog's sense of urgency doesn't match mine -- I don't really care if a large dog is walking by the house.
But whatever was bothering Jenny that night seemed very urgent. She was much more agitated than I could remember having ever seen her. Her body language was clearly telling me she wanted to herd me out of my bedroom; her tail was up in a manner that indicated something was urgent. I groggily thought, "skunk in the yard?" Yet I could tell this was much more serious for Jenny than another animal in the yard. I reluctantly got up, opened the bedroom door and she charged out, but again, seemed interested in herding me with her rather than leaving me behind.
On autopilot I trudged downstairs, through the kitchen to the backdoor and the dog pen. As I opened the door Jenny burst outside, her barking more frantic. It took me a moment to make sense of what I saw as I stood slack jawed in the back door. Orange flames and sparks were shooting out from the other side of my neighbor's house; I realized that the house on the other side of them was completely engulfed in flames. As Jenny charged around the pen barking her warning I ran inside and called 911. Then I called Jenny in and ran out the front door in time to see a young man charging onto the porch of the burning house and checking for occupants. Thank God the owners of the house were not home sleeping.
Once the neighborhood was up, the fire truck arrived and Jenny calmed down. She seemed to sense that her duty was to warn everyone and having accomplished that, her work for the night was done.
This to me is an example of ultimate watch dog behavior; when something is really wrong the dog persists in giving warning until humans take action. In fact, this is a level of behavior I wouldn't expect from the average dog; my Miniature Schnauzer who was loosing his hearing and vision simply slept through the whole thing.
Watch dogs do not attack people. In the best possible circumstances they clearly warn us when something is out of place. More often, they inform us of everything that is changing in their environment. For a dog this includes the neighbor's cat hanging out in the yard, the strange dog whose dog tags they can hear clinking from a block away, the person at the front door, a skunk in the yard etc. A watch dog ideally has a different tone of voice for these different kind of incidents. They are not dogs that bark just to bark or bark in a similar way at everything and everyone. "Barkers" will be the next topic in this series on dog behavior and we'll consider how barkers are, in their own ways, sometimes also telling us something.