>A trademark is typically a name, word, phrase, logo, symbol, design, image, or a combination of these elements. There is also a range of non-conventional trademarks comprising marks which do not fall into these standard categories, such as those based on color, smell, or sound.

I'd be surprised if Rovio doesn't consider the general appearance of the birds as a trademark. If you see a game in the app store represented by an angry bird like image, that's clearly going to create consumer confusion about the origin of the game.