Bird Dog is the nickname given to a Cessna L-19/O-1 high-altitude propeller aircraft that flies over the AO during missions. Due to its high elevation and the quiet nature of its engine, these observation planes are generally able to monitor an area without the enemy being aware of their presence. It has some limitations however: anything but large groups of enemies are very hard to detect at that altitude and due to its light and agile nature the plane has to peel off at any sign of hostile intentions by ground troops.
Bird Dog is the only support that the team doesn't have to call for it to be active, albeit passively active, or to deliver some sort of result. If Bird Dog is active over the AO and it sees enemy movement, it will communicate this to the team via the TOC channel immediately. The plane can also be requested to perform low-altitude overflight in order to spot hostile assets in the AO.
The Bird Dog can also act as a re-transmit station in order to either boost the signal strength or to gain contact with the TOC (due to the AO being so far away that the radio can not reach the HQ without a re-transmitter) due to its favorable location above ground.
You will hear the sound of a plane flying over-head when called in and when it is finished a message will be displayed: "Random Callsign, this is Random Callsign, be advised we have an (asset) at grid (number, name) with (amount) bad guys hanging around."