Theme 2-21 is one of the first levels the Foreman Pig appears. This is also the first level with a Foreman Pig boss, known as Mr. Moustache.
Shoot the White Bird's Egg at the leftmost Corporal Pig. Use the next two birds (one Red and one Yellow) to thin out the wall to the left of the next pig. Use the second Yellow Bird to destroy the floor underneath the next two Corporal Pigs, which will kill not only them, but the Foreman Pig underneath it as well. Finally, use the Black Bird to finish off the last Corporal Pig. The unused White Bird awards the player 10,000 extra points.