· Gardenality.com's Answer
Years ago, we had this very same problem at our nursery. We put up bluebird houses near where our customers parked their cars. The bluebirds would attack the mirrors of cars and poop all over the side doors.
I looked into and found out that male bliuebirds are very territorial. They peck in the mirror because they think their reflection is another bluebird invading their territory.
It doesn't make sense to park your truck elsewhere or to cover the mirrors, so, the best thing you can do to stop the problem is move the bluebird house to another location on your property, away from where you park your truck or other vehicles. If the birds have eggs in the nest or are raising their young, wait until the babies have left the nest before moving the bird house. Maybe cover your mirror with a cloth in the meantime?
Hope this info helped.