Meet the new, angriest of the Angry Birds: Mighty Eagle. Via an in-app purchase (with real money) of a can of sardines, you'll be able to summon this new bird for a one-time shot at clearing an entire level. If you want to make use of Mighty Eagle again, you can try going back to the previous level where he was used and clear it without him. Then, voila -- you can use him again in a later level.
Check out the video introducing Mighty Eagle in the 2nd half of the post.