send the third bird to crash through the boxes and destroy the last cages.
this bird should be aimed low at the box to the side of the structure, tapping to fly through it and take out the beams holding the structure up, collapsing part of it.
the next bird should be aimed higher at the cage resting on a pole to collapse the rest of the structure, leaving one cage remaining at the base.
use the last bird to hit the last bird cage.
again, the trick here is to aim the first bird for the box at the base of the structure to fly through collapsing the first two parts of the 