The first Red Bird should hit near the top of the highest horizontal wood block. The block should move aside due to the weight of the stone boulder, and the boulder should crash through the three small pigs. The rest of the structure should destroy one of the lower horizontal woodblocks, allowing the boulder to crush the two large pigs.
Tip: It will be easier to push the stack when you hit the top of the tall and thin vertical stone block which is just slightly lower than the highest horizontal wooden block. This approach is very easy to repeat, but for high scores, you need to destroy as many blocks as you can. Another interesting approach is to come in from the top by destroying the glass blocks at the top of the stack. This will cause the vertical blocks below to open the way for the boulder to fall and crush the pigs.
In older versions, the two large horizontal glass blocks near the base of the structure are replaced by wood.