It's may be very hard to kill all pigs with Blue Birds. Fortunately, it has some TNT to damage the fortress, but it's still hard.
Fire a blue bird towards the top left pig, splitting a little over half way there. The goal is to pop as many kernels as possible and push TNT into the gaps in the stone structures. This requires quite a bit more luck than skill, but keep trying and you’ll get it eventually! You can 1-bird this, but it’s also possible to 3 star using two birds.