Backward Compatibility

• To unlock the simplest hidden achievement, Backward Compatibility, open up any level. Instead of dragging the slingshot back and releasing birds toward the play field, drag to the right and fire your birds off-screen to the left. Fling 10 birds backwards in this manner to earn the achievement.
  
  

Space Invader

• To win the Space Invader achievement, you need to launch a yellow bird into space, which you can only do on certain levels. Open level 6-14 or golden egg challenge 11 for the easiest time. Drag the launcher down to send a yellow bird high up, then tap shortly before it begins falling to use the yellow bird's secondary launch. If you get it high enough, you'll win the achievement.
  
  

Wilhelm Tell

• The trickiest secret achievement, Wilhelm Tell, requires you to hit one bird in mid-flight with a second bird. To make it easier, pick a level that offers larger bird types to increase your chance of a hit. Lob your first bird in a slow arc without firing it too far to the right, then quickly shoot another to try to hit it. A successful collision will win you the achievement.
  
  

Seasons and Rio

• In addition to the achievements in the main game, both "Angry Birds Seasons" and "Angry Birds Rio" also have hidden achievements. In Seasons, the only secret achievement, The Grinch, requires you to smash 1,000 presents over the course of your game. Rio has three hidden achievements. Explosive Temperment unlocks when acquiring the black bird during the course of the game. Stellarvore requires earning a perfect three-star rating on every stage. The final achievement, 6 Million Dollar Bird, unlocks after earning a cumulative score of 6,000,000.