What causes a sparrow to peck at windows?

For the past month we have had a sparrow (I think it's a song sparrow) pecking at the windows of our house. When it first started, it pecked and fluttered continuously for almost a whole day straight. Now it starts at first light and continues sporadically throughout the day, sometimes until almost dark. It usually pecks at the small windows by our front door. It has chipped off the paint on the sills and pooped all down the wall under the windows. I tried covering the windows with paper. That didn't work. Then I tried putting some decals in the windows. That didn't work either. I thought maybe it was seeing it's reflection, but it also has started jumping around on windows that have screens. Is this bird just crazy, or what?