Our strategy for Birdday Party (Birthday Party) level 18-15 is to fire the Yellow bird horizontally, just below the stone blocks on the far left. Be sure the wooden arm breaks. Next, loft the Orange bird as far as possible so its expansion destroys the right side of the level. Finally, drop the bomb of the White bird just right of, but as close as possible to, the tower on the left side. The score in the video below is 144,860.
Did you know there is a secret achievement on this level? Click the link for the steps required to unlock it.