Why do cardinals peck at my vehicle’s side mirror?

Kenn and Kimberly:

Cardinals, like many other birds, defend a territory during the nesting season to make sure they have enough space and food for themselves, their partner and their young. If a male cardinal sees another male cardinal in his territory, he chases the trespasser away. Unfortunately, he can’t tell the difference between a real cardinal and a reflection in a mirror, so he might spend hours trying to chase away this false intruder that can’t be intimidated. You might try covering the mirror when you’re not driving your car so this defensive cardinal can go about his business.