The air compresses and moves out of the way.

Assuming the bird does not move, The back of the bus will hit the humming bird because there isn nothing to apply force to the bird and move it. But if you are sitting on the bus, It will look as if the bird hit the back because the relative velecity of the bus to you is zero and the R.V. of the bird to you will be be negative making it look as if it is moving backwards instead of you moving forward.

Tie a ping pong ball to a string and attach it to the roof of your car. It will swing back .