Star Trek: 10 Secrets Of The Klingon Bird-of-Prey
As noted in the intro, the Klingon Bird-of-Prey was initially intended to be a Motion Picture-era redesign of the Romulan Bird-of-Prey from Star Trek: The Original Series.
Early drafts of what would become Star Trek III: The Search for Spock, in fact, featured the Romulans as the film's villains and fittingly contained references to their ship as a Bird-of-Prey. However, as the script developed, the Romulans were swapped out for Klingons and the once proprietary Bird-of-Prey became a stolen one.
Ultimately, as seen in the finished film, the Bird-of-Prey in The Search for Spock was simply a Klingon Bird-of-Prey, with any mention of its origin or its theft from the Romulans left completely omitted.
There was, however, an in-universe explanation. According to writer producer Harve Bennett:
I didn't change their ship, because I remembered a piece of trivia that stated there was a mutual assistance military pact between the Klingons and the Romulans for an exchange of a military equipment.
Bennett's explanation became de facto canon for decades, with many fans believing the Bird-of-Prey was simply the product of the Klingon-Romulan alliance, similar to the use of Klingon ships by the Romulans in TOS' "The Enterprise Incident".
Unfortunately, this pretty solid bit of "fanon" was eventually negated when Star Trek: Enterprise established the existence of Klingon Birds-of-Prey operating in the 22nd century, long before the much vaunted alliance. It was a cool idea while it lasted, though.