Bourne and Marie seeks refuge in Marie's ex-boyfriend Eamon's house.
The Professor arrives there in the morning.
Bourne grabs Eamon's shotgun and starts looking for The Professor in the field near the house. At first Bourne doesn't know where "The Professor" is. He fires a shot in the air. Birds fly. Professor leaves his current position. Bourne sees The Professor and shoots him.
What was the trick behind shooting in the air? And why did The Professor move?