Most common in early spring or during the start of breeding season, and due to raging hormones that cause aggressive behavior, birds banging on your window can be a most disturbing occurrence.
The noise itself becomes nerve-racking, and concern for the bird may be secondary among some home owners. Most common are robins, bluebirds and cardinals. These smaller species are defending their habitat or nesting territory against intruders, real ones or imagined! At times they become so overly involved with attacking the rival bird, it's to their own detriment. Cats, dogs, even humans can walk right up and snatch them because they're so busy with their own reflection!
Window decals that are use to stop strikes or bird crashes at windows are ineffective in this situation. The goal is to totally block the reflection from the outside of the window. A few easy solutions follow:
Completely cover the window from outside with newspaper. If the paper is placed on the inside, birds may still see their reflection. It may not look so pretty, but just a day or two should alleviate the problem.
A mixture of powdered milk and water sprayed right on the window provides a non-reflective coating that washes off easily. Using a stronger concentrate than for drinking, simply use a spray bottle to apply. Give this method a few days until the attacking bird stops the aggressive behavior.
Attach mono-filament lines from the top of window about 12-18 inches apart. Use washers as weights to keep the strings hanging vertically. Birds will usually avoid anything that messes with their wings!
Soaping windows will also provide a non-reflective coating-but may require a bit more work for removal.
Remember, Window Alerts or decals will not help in these situations! The goal is total blockage of any possible reflection.
