As I was driving into a lawyer's office this week for a closing I noticed a couple of the cars had some accessories that would not be considered 'Factory Originals'. I found out that the bags were on the cars to protect the mirrors from the birds.
This precipitated a 'Google' to get additional information.
Birds attack shiny surfaces because they see their own reflections. In the case of a window, the bird does not see through the glass: it sees a reflection of the outside, possibly trees and the sky, and another bird of its own species. The real bird does not recognize itself and mistakes the reflected bird for a competitor for territory or mates. It attacks the image in the window, trying to drive the other bird away.
Of course, this is a fruitless exercise. The bird reflection will always be there, forever inaccessible behind an invisible barrier. The real bird often returns many times, compulsively launching itself at the window. If it is very aggressive, it could damage its beak, but most often it just wastes energy and time.
It's mating season and the males are looking to keep away the competition.
The girls in the office got tired of coming out and seeing the windows covered with droppings and the side view mirrors on their cars chipped with 'beak marks'
At least with the baggies they need only spend a couple of minutes covering and uncovering the mirrors when they arrive and leave.
This particular lawyers' office is located near water company property so there is an abundance of wildlife, and obviously some jealous suitors.