Star

The player receiving 2 stars for almost completing the level perfectly.

Stars are a fundamental element in the Angry Birds series, present in all levels. Stars are used as a format of a ranking system. In each level, stars can be earned by completing a level. With one being for simply completing the level, two for finishing the level while also having a good score, and three for finishing the level while also obtaining a hard-to-obtain score for that level, or utilising a complex tatic to finish off the pigs using fewer birds, and with more damage to the structure itself. The difficulty for obtaining a three-star ranking varies with each level.

Sometimes you may see the stars appear also with a new highscore.

Format

3 stars in Angry Birds Rio

When a stage is first unlocked, there is no written score or stars collected. As stated above, to collect stars, the player must finish the level, and based on his/her performance, he/she will be given a mininum of 1 or maximum of 3 stars for the level. If the player is able to do this for all of the levels offered in an episode, a Golden Egg is given to the player. These variants of Golden Eggs are the most difficult to obtain.

3 stars

In the original Angry Birds the Three-Star Golden Eggs unlock a minigame that if it's functions are done in a specific way, he/she will receive a Golden Egg Star. Angry Birds Seasons' Three-Star Golden Egg levels are actual levels that are played like any other level in the series. In version 2.0.0 of regular Angry Birds and version 2.5.0, the star format changes to that of Angry Birds Rio's. 

Current stars in v3.3.0+ (2.2.0+ with Retina Display)

Trivia

• Many other physics-based iOS games, such as Amazing Alex, Cut the Rope and Save Toshi use the same, or simliar 3-star system. However, the former allows the player to end the level with a zero-star ranking, and the latter allows players to get a three-star-plus ranking.
• Reseachers found that before the release of the original Angry Birds, only 20% of all games were rated with the 3-star score system. After the release of it, nearly 80% of all games were rated with the same or similar scoring system. (Similars are the likes of Asphalt 6: Adrenaline and Asphalt 8: Airborne, which placing was rated with 3, and additional objectives were 2, meaning that this is a 5-star score system.)
• In the Angry Birds Board Games, there's a star piece.
• In Angry Birds Telepizza, pepperoni slices are awarded instead of Stars.
• Along with the stars, a new highscore appears with Red very happy in Angry Birds, Angry Birds Seasons, and Angry Birds Rio.
• In Bad Piggies, a certain amount of stars are given:1 star if you complete one of three tasks, 2 stars if you complete two of three tasks, and three stars if you complete all tasks on the top of the screen.
• In Angry Birds Epic, the same three stars are rewarded in the levels, but the three star formula is different in the Magic Anvil and the Golden Caludron. At first, you may roll the die which will give you only one star for regular, but if you upgrade the Magic Anvil and the Golden Caludron, you get more stars per roll. (example: One star for regular, two stars for golden, three stars for diamond) There is also a different formula in the Golden Pig Machine - if at any chance you roll the Golden Pig Machine, the Golden Pig Machine spits out a legendary item! If hit, it will show three rainbow-colored stars instead of the original three yellow stars, and it will show the description of how you can get another legendary item but either as a shield or a sword. A fanfare will also sound if a star is hit.
• In Angry Birds POP! it is possible to get zero stars in the level as long as the objective is completed.
• Most Angry Birds (slingshot games), two-star score is usually 10,000 points lower than three-star, makes the player getting only one star in most levels.
• It is impossible to fail if you already pop all the pigs in the level.
• Angry Birds Stella is the only game in Angry Birds Series to show the target score in the level.

Angry Birds Friends

Bronze 100, Silver 200 and Golden 400 stars.

You can get a star background your avatar in Angry Birds Friends. To get a bronze, get 100 stars. To get silver, get 200. For gold, get 400. For platinum(with a diamond), get 600.