Angry Birds developer Rovio has good news for fans of its game for mobile phone and tablet PC: Angry Birds Christmas will be a free update to Angry Birds Halloween, and will be out as soon as the update is approved by Apple.

Posting on Twitter, Rovio described Angry Birds Christmas as a "free update". And following criticism that the separate, pay-for Angry Birds Halloween game didn't offer anything new to fans of the original Angry Birds game, Rovio said: "Halloween was separate because we need to keep app under 20Mb. Christmas is coming to Halloween."