Layout
In levels 1-4, you have three green pigs to defeat, hidden inside a tower built out of blocks. Two of the pigs are sitting on the ground surrounded by wood blocks, on top of which sits the third pig. Every level of the tower is built out of glass blocks except for the lowest level, which consists mostly of wood pieces with a couple of stone blocks standing upright, one on either end of the tower.
Birds
You start the level with four birds, three blue ones and a a big red one. When you fire any of the blue birds, tap the screen to explode the bird into three birds that are strong against glass blocks, but average against wood and weak against stone. The red bird doesn't have any powers of its own. It simply relies on its own size and momentum to destroy the blocks or pigs in its path.
Attack
Pull back the first blue bird in the fling shot, aiming it for the second level of the tower, the first one made out of glass. Let go of the screen to fire the bird. As it nears the tower, tap the screen to explode the bird into three to create a greater impact. The birds will destroy the first few glass pieces at least, causing the rest of the tower to crash down, destroying the pigs hiding inside. If any pigs survive, fire the rest of the birds on the same trajectory.
Tips
Don't aim the blue birds for the top of the tower. Aim them low to cause the greatest damage and have the tower fall down and score the most points. If you aim properly, you should not have to use all of your birds. For each remaining bird that you don't fling, you'll win 10,000 points. It is possible for you to beat the level using only one bird, which should easily get you three stars.