It’s called image fighting.
Every spring we frequently address the question “why is this cardinal (or some other bird) trying to get in my house?” Cardinals are the most common bird to image fight, but it can be any number of species including Eastern bluebird, American robin, Eastern towhee, and even Pileated woodpecker. Cardinals are the most common, likely because they often nest in shrubs around the foundation of a house putting windows and glass doors very close to their comings and goings.
Image fighting refers to the sometimes constant presence of a bird fluttering at and making contact with a window, or many different windows of a home. Sometimes it will be the side view mirror of a vehicle. During the breeding season a bird’s sense of territoriality is heightened. A bird will pass a window and detect the movement of a bird, that appears to be the same species, and it will fly up to the window seeing what appears to it to be a competitor. It’s the result of a very strong instinct that hasn’t been able to adapt to a man-made object. This activity can go on for months, and well beyond the breeding season if unaddressed. The bird rarely injures itself seriously but it becomes an incredible annoyance to you and keeps the bird extremely occupied with a futile activity when it could be doing something much more important.
For your sake as well as the birds we recommend stopping this behavior as soon as possible so the bird can resume normal activities and keep you from getting very frustrated and contemplating doing harm to the bird. The best way to stop this activity is to block the source of the reflection for a period of no less than a week. This can be achieved by putting up a piece of cloth or cardboard cut to the size of the pane and affixing to the glass, thus removing any possibility of the birds seeing its reflection. It MUST be applied to the exterior. Putting it on the interior can actually make the reflection more pronounced. Other things like a couple of inflated balloons tied to a piece of string taped to the glass works very well, again, on the exterior. The balloon moves around with the slightest breeze keeping the bird from wanting to approach. Again, try this for no less than a week. Repeated applications may be necessary. Usually the behavior lessens or stops after the breeding season concludes.
If you are having an issue with birds image fighting on the side view mirror of your vehicle try putting a bag or an old pillowcase over the mirrors while you’re not using your car. Try one of these methods to help your birds get back to the business of raising young.