bird is moving (todo: reset positioned when it has moved off screen)

Change-Id: I37c5d0252da2e6ec04b1a65211bd7d9f469802c9
bird is re-positioned when it has moved off screen

Change-Id: Icff77bf4c0066fba0bfbcef7683de38d6177eb0d
some refactoring

Change-Id: I513fcf14710115aaf7201990dc3e14093c08595d