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A 22nd century Bird-of-Prey.
A Bird-of-Prey (aft view) chasing Enterprise.
In the 22nd century, the Romulan Star Empire fielded a warship known as a "Bird-of-Prey", so named for its distinctive bird-like hull form and for the bold pattern of an alien bird painted on its ventral hull. (TOS: "Balance of Terror") The ships were equipped with advanced weaponry, as well as a cloaking device.
The Bird-of-Prey was first encountered by Humans in mid-2152, when Enterprise accidentally entered a minefield surrounding a planet claimed by the Romulan Empire. Two ships of this type were present in the system and confronted the Enterprise, forcing it to leave. (ENT: "Minefield")
- The fact that the Birds-of-Prey in the 2150s were equipped with cloaking devices constitutes an apparent continuity violation, because Spock's comment gives the impression that cloaks were apparently unheard of before "Balance of Terror". However, this is Spock's own personal notion based on what he knows, and does not necessarily reflect the totality of the Federation's expertise on Romulan equipment. Alternatively, the fault could be said to lie with previous episodes, because "Minefield" was not the first time a cloaking device was seen on Enterprise. Another possibility is that the cloaking device used in "Minefield" was of a different design than the cloaking devices seen in the 23rd century. This would be consistent with Kirk's line about it being their "new cloaking device" in the episode The Enterprise Incident, which could (if you use your imagination) imply that there have been other cloaking devices in the past.