Romulan Birds-of-Prey were starships used by the Romulan Star Empire in the 22nd century. Green in color, they were so named due to their distinctive avian-like appearance. They were equipped with disruptor banks, as well as advanced cloaking technology.
These vessels were first encountered by Starfleet in 2152, when the Enterprise NX-01 inadvertently encroached upon a Romulan-claimed star system, which was protected by a field of cloaked mines. Although Enterprise's quantum beacons proved successful in penetrating the mines' cloaks, they were useless against the Birds-of-Prey's more advanced cloaking devices. (ENT: "Minefield")
Ships of the class
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. See also Earth-Romulan War.
The appearance of the 22nd century Bird-of-Prey, at face value, constitutes an apparent visual continuity violation as well, in that Lt. Stiles mentioned in the same episode that during the Earth-Romulan War, Romulan ships of the time were painted like giant birds of prey (hence the name). Some fans have speculated that the 22nd century Birds-of-Prey with their cloaking devices and overall pseudo-24th century appearance are the result of the Temporal Cold War. We can also assume that other classes of Romulan ships during the 22nd century were painted like giant birds, especially smaller, more easily manufactured cruisers, making a majority of their unseen fleet possibly able to validate the comments of Stiles. It is also possible that the Romulans did not adopt the Bird-of-Prey markings until the Earth-Romulan war had actually begun.
Several staff writers on Star Trek: Enterprise – among them Producer Mike Sussman – believed that 22nd century Romulan ships should not have been equipped with cloaking devices, in keeping with "Balance of Terror". Season Four showrunner and Co-Executive Producer Manny Coto decided that Romulan ships would not have cloaks in any subsequent encounters. Birds-of-Prey were seen in Star Trek: Tactical Assault as light cruisers.
According to the novel The Good That Men Do, these cloaking ships were one-offs. Romulan stealth technology was still in development, the cloaked mines were considered a successful application of the technology but the power requirements for cloaking a whole ship were problematic, sometime after the encounter with the Enterprise the prototype cloaking ship, Praetor Pontilus, was destroyed in an antimatter containment failure caused by its stealth systems. The Romulans believed it would be decades until a successful cloaking device could be developed.