Why do cardinals keep flying into (closed) windows on one side of house? It usually occurs during the early morning hours and occasionally in the afternoon. Is this a common occurance or something specific with the bird? (a minor theory I have is that we stopped feeding birds as our feeder broke recently and haven't replaced it--is this their way of telling us--this probably sounds stupid, but anyway) Any ideas?

I can think of two possibilities: 1. Given certain light conditions, the window may reflect the sky and other surroundings outside. Birds don't recognize that it is just a reflection of the sky and end up flying into the window. 2. Tis the breeding season. And cardinals are pretty territorial. If a male sees his reflection in the window he may think it is an "intruder" in his territory and will "attack" the "intruder".