That is actually a completely different bit of physics all together. That is the theory of gravity, the OP's question deals with the theory of relativity, with a bit of Newtonian Motion. Close, but still different.

And to answer, they both hit the ground at the same moment, as long as the drag co-efficient isn't much greater than that caused by normal air molecules (you can take out that variable all together by doing this experiment in a vacuum).

 

the bird and the car are both two independent masses. if one stops the other will continue in motion, ie, the bird would hit the front window.

since they are two independent bodies, you can disassociate them for easier comprehension... imagine two cars rolling down a hill at the same speed. one slams on the brakes, does the other keep moving? yes as it has its own momentum.

the fact the humming bird is hovering inside the car makes no difference.

 

but... humming birds are the only ones that can fly backward, so wouldnt it hit the back windshield?

 

without having a real hummingbird to do the experiment with, its really hard to say for sure- yes, if it just hovered and didn't try to adjust its position relative to its environment- it probably would hit the windshield, but since a hummingbird is a living creature, it has the ability to sense its position relative to things around it and change the way it is flying, and since your brakes have limitations (you can't stop your car instantaneously) the rate of deceleration will have an effect on how much the hummingbird can compensate for the change of speed of the vehicle going on around it.

No way it will hit the back window though, not gonna happen...