Danger Zone D-4 is the fourth level in Danger Zone. The objective of level D-4 is to use the birds to eliminate all the pigs. Get at least 90,000 points for 3 stars.
This is a rock fortress, so it's hard to destroy the rock. And you have an Ice Bird as the last bird.
Send the Ice Bird into one of the joints of the cross. Launch a Black bird around the other side of the planet and through the weakened structure. If more pieces remain, use the Ice bird to finish it.