will it move backwards because of the air moving forward.

Yes, which is presumably something the bird can perceive, and make adjustments to. If the bird is determined to stay in one place (which is an absurd concept in and of itself) then would this be sufficient? 100mph may be too fast or too slow. If the car was going 1mph then would the air rushing forward cause the bird to "stop" or "move backwards"? The bird is moving itself independently of any other motion. Therefore the OP is basically asking if a helium balloon would move backwards or forward. Or he's asking a purely theoretical question about "a weightless object suspended above a car moving at 100mph" but that itself is a meaningless question without further context about how it's hovering. In the case of a balloon, it moves backwards because of density. In the case of a bird, it would depend on a lot of other factors. Size of the bird, weight of the bird, mechanics of its wings (which are fairly unique for hummingbirds), etc.

I propose (and I'm QUITE certain I'm right) that OP is getting confused with the helium balloon experiment in which a helium balloon is displaced by the more-dense air, and moves counter intuitively.

Very possible, but there isn't enough background to know. If a hummingbird is a meaningless object then it's entirely probable he is thinking of that experiment and confusing the mechanics of a balloon with that of a bird, however, if that is the case then the question remains the same: Does the bird move forward or not because of the air. You could magnify this argument to the speed of light and imagine an instantaneous deceleration to "0mph" (relative to the Earth) and in doing so can imagine how the air itself is moving 100mph and begins to collapse on the windshield. This creates force which propagates towards the bird. This would have a Newtonian effect which would "propel" the bird towards the glass, and would be similar to the wind. Hummingbirds are used to that and would respond, just like a balloon or a helicopter would respond. The response of the bird, how its wings function, its size, terminal velocity (which is probably a hell of a lot less than 100mph), etc., would all factor into the effect. Strictly speaking the air would push it forward, but this force could also interact with its wings (which can beat up to 70times per second) in such a way as for the bird to move backwards.

So is the question about what happens to the bird? Is it about what way the air pushes an object with a density greater than helium?

NOT backwards "because of the air in the car"

You're probably correct, but I don't know anything about how hummingbird's wings interact to "hover stationary" or would impact such a force of air would impart on it. Would it matter if the cabin was pressurized? What if the crash didn't rupture the cabin? Is this question physically about a car hitting a wall, or simply a car theoretically decelerating at or above the speed of light.