Perhaps when the car is parked up cover the mirror with an old sock and ensure the back window or the car is covered. Also many birds will fly into/peck at windows, you can buy stickers etc to stop birds doing this (the RSPB will have items for sale which will also put money towards a good cause!) However, its is very common for birds to do this at this time of year, it will stop soon, and its very rare for a bird to become seriously injured from this behaviour! Best of luck!