Birdday Party

Birdday Party is a bonus Angry Birds episode. It celebrates the 2nd anniversary of Angry Birds in 2011, its 3rd anniversary in 2012, its 4th anniversary in 2013, its 6th anniversary in 2015, and its 7th anniversary in 2016, taking place in a fantasy world filled with candy, birthday cakes, and other sweets, as opposed to the realistic environments seen in previous episodes. The episode itself serves as the eighteenth, nineteenth, and twentieth themes of the original game, and marks the 300th level mark with Cake 2 Level 15.

Formerly, this episode was considered the seventh of the game. However, it no longer uses the regular world numbering, instead opting for cakes with numbers on top. Surf and Turf is now what continues from Mine and Dine, and Bad Piggies continues from that.

Storyline
The following is from The World of Angry Birds Official Guide, as told by the Mighty Eagle:

HAPPY BIRDDAY TO YOU! It was Chuck's clever idea. Taking a leaf out of the hog's book, the Flock disguised the eggs as a birthday cake. What a half-baked plan, birdbrain. Dressing the nest up as food? It just made King Pig hungrier than ever.

New Birds

 * Bubbles
 * Stella

Cake 4

 * 1) Theme 1 Cake 4
 * 2) Theme 2 Cake 4
 * 3) Theme 3 Cake 4
 * 4) Theme 4 Cake 4
 * 5) Theme 5 Cake 4
 * 6) Theme 6 Cake 4
 * 7) Theme 7 Cake 4
 * 8) Theme 8 Cake 4
 * 9) Theme 9 Cake 4
 * 10) Theme 10 Cake 4
 * 11) Theme 11 Cake 4
 * 12) Theme 12 Cake 4
 * 13) Theme 13 Cake 4
 * 14) Theme 14 Cake 4
 * 15) Theme 15 Cake 4

Cake 3
 Theme 19-1 Theme 19-2 Theme 19-3 Theme 19-4 Theme 19-5 Theme 19-6 Theme 19-7 Theme 19-8 Theme 19-9</li> Theme 19-10</li> Theme 19-11</li> Theme 19-12</li> Theme 19-13</li> Theme 19-14</li> Theme 19-15 </li></ol>

Cake 2
<ol> Theme 18-1</li> Theme 18-2</li> Theme 18-3</li> Theme 18-4</li> Theme 18-5</li> Theme 18-6</li> Theme 18-7</li> Theme 18-8</li> Theme 18-9</li> Theme 18-10</li> <li>Theme 18-11</li> <li>Theme 18-12</li> <li>Theme 18-13</li> <li>Theme 18-14</li> <li>Theme 18-15</li></ol>

Trivia

 * Before the episode colors were removed in an update, the representative color they used in Birdday Party is lime.
 * This is the first theme in Angry Birds that takes place entirely within a fantasy area and is the only world where the theme numbers are arranged backward (Page 1 is Theme 20, Page 2 is Theme 19, Page 3 is Theme 18)
 * This is the first Angry Birds episode that involves an anniversary.
 * All of the updates involving this episode were released on December 11 every year, the same day Angry Birds was first released.
 * This episode had the fewest amount of levels in the game, with only 15, before the extra levels were added on December 11, 2012.
 * The title is a reference to the saying "Birthday Party" when the birds are having their birthday party.
 * This is also considered the first of the episodes to have its pun-based title on a Bird, rather than a Pig, Eggs, or eating.
 * This is the first time the Bubbles the Orange Bird and Stella the Pink Bird appear in the original Angry Birds game. Stella, though, never had her sound effects from Angry Birds Seasons when she was updated in Classic. That issue was fixed in Version 3.1.0. (Mar 7th, 2013, when Bad Piggies was updated with 15 new levels and also when the app was free until the 14th).
 * Updating this episode not only provided players with all the episodes that are now unlocked without having to clear the first episode, but all the cutscenes were updated to the design shown in the Google Chrome Version of Angry Birds.
 * This episode was launched for the game's second anniversary and then proceeded until the 8th one.
 * This is the first Angry Birds episode that is based on food with the second being Utopia in Angry Birds Space.
 * On level 15 of cake 3, there is a rainbow-colored slice of cake. Getting it will display a new achievement called "Cakemonger".
 * This episode received an update on December 11, 2012, that shows 15 more levels, and chapter 19 is moved from Bad Piggies to this episode. Also, 15 more levels of the Bad Piggies episode have been added.
 * Instead of a theme number, on the pause menu, there is a cake with 2, 3, or 4 candles on it, depending on the year the levels came out. Also, the first level of cake 2 has one more Bubbles and the final cake 2 level has King Pig taking place of the Minion Pig before the 12/11/2013 update came.
 * This update shows Matilda's and Hal's new sprites. It also shows the corpse sprites for the Birds from the Rio, Volcano, and Chrome games.
 * Before the 2nd update of Birdday Party on Theme 18-15, King Pig was nowhere to be seen and got beaten. Before the 3rd update on Cake 3 Level 15, King Pig was also nowhere to be seen and also got beaten. The 4th update corrects this, while on cake 2 level 15 King Pig appears as now the final level of Birdday Party. On Cake 4 Corporal Pig is defeated, on Cake 3 Foreman Pig is defeated, and Cake 2 all the pigs including King Pig are defeated.
 * Birdday Party was updated with 15 new levels to celebrate Angry Birds' 3rd anniversary in 2012. These new levels feature the Pink Bird (appearing for the first time in the original Angry Birds), changes in Birds' sprites, and new features like TNT Cakes. These levels have the number "19" for the theme number, so the Bad Piggies episode was renumbered accordingly. Also, there is a new hidden golden egg in level 19-14/cake 3 level 14.  Also, the update now includes the shaking background when the Black Bird, the White Bird's eggs and TNT crates explode, and when the Orange Bird inflates.
 * Of all of the Classic episodes, the new birds from Seasons appear in this episode.
 * In world 19/cake 3, Bubbles only appears in Theme 19-3.
 * With V. 3.1.0, Stella has her voice and sound effects from Angry Birds Seasons fixed in Angry Birds Classic.
 * This is the second episode to have background music with the first being Mine and Dine.
 * With V.3.4.0, the numbering is now in cake depending on which anniversary, such as the 2011 anniversary being labeled as Cake 2, 2012 as cake 3, and 2013 as Cake 4. However they are still labeled as Themes 18, 19, and 20, as the numbering for Bad Piggies was updated to accommodate this format, with its three themes being re-branded Themes 21, 22, and 23 instead of 20, 21, and 22.