Angry Birds (video game) crates and rocks are incorporated into the structures, and power-ups may be used to enhance the birds to attack hard-to-reach pigs. Also in the levels, players can unlock bonus levels by collecting golden eggs. There are several different types of birds used in the game, distinguished by their color and shape. In the earliest levels only the basic red bird is available. As the player advances through the game additional types of birds become available. Some birds are effective against particular materials, and some have special abilities that may be activated by the player while the bird is airborne. For
Angry Birds (video game) for any unused birds. Upon completing each level, players receive one, two, or three stars depending on the score received. Players are encouraged to repeat any previously beaten levels in which the full three stars weren't awarded in order to fully master them and earn the full three star rating. In early 2009, Rovio staff began reviewing proposals for potential games. One such proposal came from senior game designer Jaakko Iisalo in the form of a simulated screenshot featuring some angry-looking birds with no visible legs or wings. While the picture gave no clue as to what type of game