There have been several types of vessels referred to as a "Bird-of-Prey". Although the term was apparently originally a Romulan name, it was also adapted by the Klingons. The Bird-of-Prey was the Romulan equivalent of an escort. They were not heavily armed, instead they specialised in surprise attacks by decloaking, attacking, recloaking and retreating before the enemy can respond. (Ship Recognition Manual, Volume 5: Starships of the Romulan Star Empire)