What does it mean when a bird hits your car?

The bird was confused and ran into you or it was on a a suicide mission.

Not all naturally occurring phenomenon are a"sign", more often than not it is just the normal interaction of nature's creatures with us and our "things".

Most likely the bird was as stated above confused or startled, it may have thought the glass was not there due to its reflective qualities or it just may have been unlucky. Birds are really not mentally evolved enough to be despondent enough to try to do away with themselves. That said, however, if the type of bird is of personal significance to you personally, then you may take it as a sign to "watch where you are going"; "pay attention" or that there may be danger in your future.