Instructions
Talk to Tsunade during mission 11. She will tell you to retrieve the Shisune letter from a bird.
Travel to Konoha village and climb the gate near the entrance. You should be able to see the bird from this area.
Press the "mode select" button (which varies depending on which system you are playing on) until you are in "shuriken mode." Aim at a spot slightly ahead of where the bird is flying and throw as many shurikens as you can. The bird will fall to the ground when you hit it.
Climb down into the village where the bird fell. Walk over to its unconscious body and press the "action" button. Pick up the bird and retrieve the Shisune letter.