The sixth level is slightly tricky. This should be doable in two hits (for all three stars). Unfortunately, I was unable to capture all of the pig popping in the video, but I will explain them.
As you can see in the video, two pigs are popped right away by the first bird. However as a result of chain reaction, three more pigs are popped. That portion, unfortunately, is not shown in the video. It may take a few tries to get the right angle and the right hit.
The second hit should be used to hit the pig on the upper level, which shouldn’t be too difficult.