I'm confused as to why the series refers to both romulan ships and klingon ships as "The Bird of Prey."
Is "The Bird of Prey" supposed to be the name of a particular ship, or is it a type of ship? Do romulans and klingons just happen to use the same type, or are the two species working together? And, how did both groups come to develop a cloaking device that the Federation apparently didn't have the science to develop?
IIRC, the Romulan ship in "Balance of Terror" was never referred to as a "bird-of-prey." Stiles stated that Rommie ships from the Earth-Romulan war had birds painted on their undersides (which ENT showed that they didn't
, but that's another story...), but no Rommie ship (or Klingon ship for that matter) in TOS was ever referred to as such. It wasn't until The Search For Spock that the term was used. Of course, originally the Bird-of-Prey was supposed to be a Romulan ship before the script got changed to Klingons being the adversaries, but by that time the term and ship had already forever stuck to the Klingons.
However, the terms "bird-of-prey" and "warbird" have been so jumbled together between Klingons and Romulans that they're pretty much interchangeable these days