Good watchdog in a small package

My Scottie is the best early warning system I have owned. He alerts to anyone at the perimeter with two sharp barks, then waits. He does not bark endlessly and he does not react to every sound. He is selective and accurate. His bark is surprisingly deep for his size. He is reserved with strangers but warms appropriately once introduced. He has been an asset in our neighbourhood where we have had some security concerns.