Updated February 11, 2020 at 2:38 p.m. EST: The Verge reports that an unnamed Warner Bros. spokesman has confirmed that the studio has allowed some theater chains to update the name of the film Birds of Prey (and the Fantabulous Emancipation of One Harley Quinn) to Harley Quinn: Birds of Prey as part of a “search expansion for ticket sites."
The Verge theorizes this is due to the film's opening weekend domestic take of $33 million, falling short of the studio's $50-55m projections for the R-rated Suicide Squad spin-off. The outlet also quotes an unnamed "industry expert" who chalks up the underperformance to a lack of understanding of the film's plot, a lack of promotional material showcasing its R-rated elements, and a long and confusing title that didn't highlight the film's main character.
"Like a domino effect, a few things went wrong at once: bad marketing, bad trailers, and bad title decisions," reads The Verge's explanation.
Original Story from February 11, 2020 at 11:29 a.m. EST: Birds of Prey (and the Fantabulous Emancipation of One Harley Quinn) has been retitled Harley Quinn: Birds of Prey on the websites of some major theater chains, including AMC Theatres, Regal Cinemas, and Cinemark.
Several other major chains (including Landmark Theatres, Arclight Cinemas, and IMAX Theatres) list the title as simply Birds of Prey, with no subtitle - though it's unclear if that's a change, or how they previously listed it. That's also how Warner Bros. official social media accounts for the film list the title, both in page title and in the media on display - though neither the film's official Twitter or Facebook seem to have displayed the subtitle at any point.
The change was first noticed by Twitter user Stitch Kingdom, who posted a screenshot of AMC's website. ScreenRant later stated they had independently confirmed that the change was requested specifically by Warner Bros.
General online ticket retailers Atom Tickets and Fandango both list the film's full original title, Birds of Prey (and the Fantabulous Emancipation of One Harley Quinn). Fandango is partially owned by Warner Bros.