Aircraft using the Bird Dog callsign are typically the spotter and guide planes for ariel firefighting activities (at least here in Canada). The following is from a firefighting website:
"Bird Dog" Aircraft
Often overlooked in forest firefighting operations are the "Bird Dog" aircraft. They are called that because they act as the spotter, and lead waterbombers safely into the bombing pattern.
No specific make or model of plane exists that could be called the definitive Bird Dog aircraft; a large variety are flown by spotters around the world. What all have in common is the task. On a fire call, the Bird Dog is airborne within minutes and sets up contact with the ground-based fire boss and determines the fire location, size and nature. Once the site has been assessed by the spotter, he radios back the information to the fire boss who directs the entire land and air firefighting operation. While enroute to the fire, the waterbomber or tanker receives instructions from the Bird Dog as to circuit altitude, drop height, and exit plan. The Bird Dog leads each loaded waterbomber directly over the fire site for its drop and may continue for as many drops as it takes to quench the flames.
Often pilots rotate between the waterbomber and Bird Dog aircraft so that they are proficient with both. It is most beneficial for the Bird Dog pilot to be familiar with the waterbomber requirements particularly in the waterbombing role.