The bird in the window

I have heard stories about birds flying into windows, but I had never seen anything quite like the two birds I observed recently. I was walking by a newly finished, unoccupied house and saw them sitting side by side on the protective bars in front of a large window. One of the them was pecking angrily at his (or her) reflection in the glass as the other one watched intently. What in the world were they doing? They flew away as I walked by. About an hour later I walked by again and there they were, still at it. One of them banging his beak (that had to hurt) against the glass and the other observing. I think I have spent a lot of emotional energy on things just about that profitable, hurting myself in the process, and have watched others do the same thing.