You need to look at each level and weigh up the damage/bird ratio. Some levels can be 1 birded like 3-8 and then its just a point of getting all the pieces to break lucky for you.
Levels like 2-15 and 3-21 you need to look at the things that get you the most points and try and knock them down with the least number of birds. There is no better way to explain it. I can try to do the same for level 3-21 but sometimes more blocks break than others.
2-15 try to kill the 2 bottom pigs nearest with the bomb and also knock down the stone building in front with the offshooting bomb bird.... get the idea. if you are lucky the bird will also pop over the other side and drop on another pig....
Note that some levels can be one birded like 3-5 but i got 99k with 2 birds because 2 birds break more blocks than 10k points.
Hope that helps