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 appearance. They were equipped with disruptor banks, as well as advanced cloaking technology. (ENT: "Minefield")
These vessels were first encountered by Starfleet in 2152, when 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-Preys' more advanced cloaking devices. (ENT: "Minefield")
Ships of the class Edit
Background information Edit
In the script of "Minefield", this type of ship was described thus; "A spooky-looking craft, there's something bird-like about its design (we will later learn that this is a Romulan vessel of the era)." A subsequent description in the same teleplay referred to the ship as an "ominous-looking craft."
The responsibility of designing this style of Romulan Bird-of-Prey, the first new Romulan ship to appear on Star Trek: Enterprise, was given to Concept Artist John Eaves. He was hugely excited to be presented with the opportunity of designing a new starship for such a long-established alien race as the Romulans. He later remembered, "Immediately the consensus [in the art department] was to retro-out Wah Chang's original design for our episode." One reason why Eaves was a suitable candidate to assume this task was that he was very familiar with the history of Star Trek design. He knew the Bird-of-Prey he was designing for ENT needed to pay homage to Chang's equivalent. (Star Trek: The Official Starships Collection, issue 27, p. 10) "I didn't want to veer too far from it, but I wanted to do a phase prior to The Original Series version," reflected Eaves. "I followed his lines a little bit." (Star Trek: The Official Starships Collection, issue 27, p. 11)
Next, John Eaves opted to add further segregation to the parts of the hull, segmenting them where one section could be seen joining another. This was key to him modifying the ship, as it made the design appear slightly older, though also dynamic. The use of modern visual effects meant the updated design would not have worked with the smooth hull style from its TOS predecessor, anyway. "The VFX guys always wanted deep shadows and detail lines to help make the ship look authentic on screen," said Eaves. "Something that was completely smooth, like Chang's design was, would never make it through the approval process today." (Star Trek: The Official Starships Collection, issue 27, pp. 11-12)
One detail which John Eaves added in his early design concepts was small wing fin attachments on the outside of the nacelles. These were incorporated in an initial black-and-white sketch he illustrated while experimenting with various looks for the ship. "After I'd done a couple of versions of it," he stated, "I abandoned them. I decided I'd taken it too far past where the design architecture had been established." The nacelles in specific also underwent revision in a subsequent series of color concept images, developing to have a more threatening look, as Eaves aimed to devise a style more in keeping with the Romulans. "The nacelles on the early black-and-white drawing seemed a little bit too rounded," he explained, "so that's why they evolved into jagged points. I thought the early design had too much of a soft, friendly look for an aggressive ship. That's why they go from the rounded tips to the more bull-horn-type points that I did in the colored drawings." (Star Trek: The Official Starships Collection, issue 27, pp. 11 & 12)
As a final homage to Wah Chang's original Romulan Bird-of-Prey, John Eaves drew a colorful war-bird graphic on the underside of the altered configuration, in one of his concept illustrations. "I was trying to take his [Chang's] drawing back a little in time as well, but make it fit the new shape," Eaves recollected. Though he hoped this facet of the design would make it onto the finalized digital model, the graphic was ultimately vetoed because a decision was made that the stylized bird picture clashed with the overall design ethos which had been established on ENT. (Star Trek: The Official Starships Collection, issue 27, pp. 12 & 13) Eaves related, "Just when you think you've done the right thing… lighting strikes…. The graphic had to be removed… Ahhh, shuckydarn!!!!"  He elaborated, "In fact, it got rejected brutally. I was hoping it would make it through because it was a neat way to pay homage to Chang's ship. But I understood that it had to go, as Enterprise was set in a different era." (Star Trek: The Official Starships Collection, issue 27, p. 12) No motive for the removal was made explicit to Eaves, though.  Robert Bonchune offered, "It was rejected for no other reason than, once again, contempt for the Trek, the fans and the Original Series by… uh, 'management'… you know who they are. (Oh, and it wasn't their idea; that didn't help….)"  Even though it never appeared on screen, the bird graphic was featured on the bottom of a version of the Bird-of-Prey, which Bonchune created. (Star Trek: The Official Starships Collection, issue 27, p. 10)
Despite the elimination of the bird picture on the final design of the Romulan craft, John Eaves opted to add lots of surface detail to the ship's exterior, helping the vessel seem more realistic when it became a CGI model. The finished design appeared in a concept image dated July 2002. (Star Trek: The Official Starships Collection, issue 27, pp. 12 & 13) Eaves concluded, "The architecture was pulled back about a hundred years and the end result was just enough to still pay homage without creating a whole new ship." 
The fact that the Birds-of-Prey in the 2150s were equipped with cloaking devices constitutes an apparent continuity violation, because a comment made by Spock, in "Balance of Terror", gives the impression that cloaks were apparently unheard of before the 2266 setting of that episode. 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. Also Spock was familiar with the concept of cloaking and it's limitations which suggests that the Federation had seen cloaks before, but less effective ones. 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 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.