Angry Birds Seasons allows fans to celebrate each holiday with more slingshot fun.

Angry Birds quickly became the App Store’s number-one paid application and stayed there for most of 2010; an M.I.T. publication called the game “the largest mobile app success the world has seen so far.” My Senior Video Games Correspondent – my son (and future game design superstar) Benny credits the game’s success to viral word of mouth, “a person telling a person who happens to have a large Twitter following about the game. Several of those people combined with the easy to enter Android Market and App Store equals a lot of sales.”

Update after update followed the game’s initial release, adding features like hidden ‘Golden Eggs’ that unlocked bonus content, or the “Mighty Eagle,” a superbird capable of wiping out uncompleted levels. (The eagle must belong to a strong union – you can only call on him once an hour.) Holiday versions themed to Christmas, Halloween and the like have appeared as well; according to the company there are over 40 different versions of Angry Birds containing some 300 levels in a total.

The cross promotion between Fox's Rio and Rovio's Angry Birds was an easy fit.

No matter the level there are only two factors for the player to control: the amount of stretch applied to the slingshot and its degree of elevation – any combination of which will send the bird hurtling towards a different point on its target. (Later versions include a ‘boost’ feature that rockets a bird in mid-flight to even greater speed or distance.) According to my Correspondent, the birds’ trajectory and subsequent damage to the pigs is generated by a simple ‘physics engine’ – computer software that basically calculates what happens when one object collides with another. Console video games require a high-powered engine to calculate and create the complex interactions taking place onscreen, in real time in their three-dimensional imaginary worlds, but the simply drawn characters and 2D action of Angry Birds are nowhere as difficult to render – which is why you can play it on your smartphone.

There are dozens number of similar, downloadable physics-based games out there built around the simplest ‘ragdoll’ engines; why have none of these gone viral to the degree Angry Birds has?

According to Rovio’s Peter Vesterbacka, it’s the personal touch – both between the company and the fans, and the fans themselves: “One of the things that is very important is that no matter how big the number [of Angry Birds fans] gets, we try to reply to every email, every tweet that people send our way. So that's very, very important, and I think that's also a big reason for the success. If you look at Angry Birds, we did none of the traditional advertising or any of that. It doesn't really make sense for a 99-cent game; you can't make the numbers work.

“We did a lot of word of mouth instead. The whole game, it's all organic growth. So the seven million downloads we've done, they're all organic. And that's something [that] also tells you about the game, that it's the most social game around, because everybody's telling their friends about Angry Birds and asking “which level?” and “found any new golden eggs?” or “which birds are you using?” and all that.”

Considering the game’s popularity, there’s surprisingly little Angry Birds merchandise out there. Apart from the expected plush pigs and birds, iPod cases, T-shirts and a Mattel board game version, its highest profile appearance may have been a tie-in game featuring Blue Sky’s Rio characters in place of the usual angry birds and greedy pigs.