I don't think its that simple. Wikipedia says

>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.

You don't consider things trademarks, it's either a registered trademark or it's not.

That's not true. There's no need for a trademark to be registered or even to be indicated as being a trademark (typically by using 'TM'), though of course you do get extra rights if you do. The US Patent Office is a good resource for info: http://www.uspto.gov/trademarks/index.jsp

That bird design is to Rovio what Mickey is to Disney...