(covers information from several alternate timelines)
In 2151, the Vulcan diplomat Tos claimed that the Klingons would have sent a squadron of warbirds to attack Earth, had he not negotiated for the transport of Klaang's corpse to Qo'noS on the Enterprise NX-01. (ENT: "Broken Bow")
In an alternate reality, five simulated Klingon warbirds were incorporated into the Kobayashi Maru scenario. These warbirds were capable of cloaking, had shields, and were armed with photon torpedoes. During James T. Kirk's third attempt at the scenario in 2258, two of these vessels entered the neutral zone before locking weapons on the USS Trainer captained by Kirk. Moments later, the three other warbirds decloaked while targeting the Starfleet craft, and then began firing torpedoes as they simultaneously maintained shields. After the simulation was affected by a subroutine that Kirk had secretly inserted, the warbirds' shields were suddenly eliminated and the Klingon vessels were all vulnerable to a single photon torpedo strike each.
Shortly before, Cadet Uhura heard a transmission that 47 Klingon warbirds were destroyed by one Romulan ship (which turned out to be the Narada) when it attacked the Klingon prison planet. (Star Trek)
The warbirds in Star Trek were designed by John Eaves. In a scene deleted from the film, after the Narada is disabled by USS Kelvin's attack, a fleet of ten Klingon warbirds decloak and surround Nero's ship.
The use of the term "warbird" in relation to the Klingons, as used in ENT: "Broken Bow", was a writer's error. Soon after the episode's production, Brannon Braga stated, "We made one mistake where in the pilot we called the Klingons' ships warbirds, when in fact the Romulans use warbirds and Klingons use the bird-of-prey. There was nothing we could do about it, and I'm sure we're going to get a lot of angry fans. But it was an honest mistake." (Star Trek: The Magazine Volume 2, Issue 9, p. 20)