SAN FRANCISCO--It's no secret Rovio has made a small fortune off its sales of Angry Birds on the App Store (and other platforms), but there had been some question about the success of its dabbling in the in-app purchase market.

That question was answered this afternoon by Rovio's "mighty eagle" Peter Vesterbacka at the Game Developers Conference here. In a talk outlining the company's efforts to build the Angry Birds franchise beyond its humble beginnings, Vesterbacka announced that 40 percent of new Angry Birds buyers had purchased the 99-cent "mighty eagle" add-on, which lets users skip a level they're stuck on by unleashing a powered-up bird.

Vesterbacka did not go into detail on how those numbers trickled down to users that had purchased the game since its release, but suffice to say that the company has been pleased. That success, he said, hinged on making add-on game content that had a wide appeal. "It's bad if you make products that 2 [percent] to 3 percent of your mobile fans want to buy," Vesterbacka said.

Other tidbits revealed during the talk were that Rovio had sold more than 2 million of its plush toys. Vesterbacka described that as "a good start," while saying that it was only the beginning of the company's plans to expand the franchise.

Vesterbacka also talked up the upcoming sequel to Angry Birds, which is a tie-in with the upcoming Fox film Rio. "We didn't want to sell out," Vesterbacka said of the partnership. In the lead up to that choice, Vesterbacka had described some of the offers from other studios as "weird."

"We got approached by different Hollywood studios, and they wanted to do all kinds of weird promotions, and lots of them were not very...let's say they didn't jive with the brand," Vesterbacka said.

Being a developers conference, Vesterbacka also urged game makers not to give up in the face of not having a smash out success, citing that Angry Birds had actually been Rovio's 52nd game. "The previous 51 games, those were also great games for the devices at that time, but of course the devices at that time were pretty limited," Vesterbacka said. "If you look at the early J2ME/Brew games, the experiences were not amazing."

There was also a particularly awkward moment in the question-and-answer part of the presentation when Vesterbacka was asked what physics engine Rovio had used. "Box2D," Vesterbacka replied. The question asker turned out to be the creator of the open-source physics engine and asked whether the company would be giving him credit in Angry Birds. Vesterbacka encouraged the gentleman to come see him after the talk, as well as for other attendees to introduce themselves before asking their question.