[Aliantha]

The reason I think it's frost is that the impression is fairly clear, so the material the impression is made from must be easily removed. ie, frost from glass.

Having said that, if it's just a dirty window, the it must be just dust and not dirt. For dirt to stick to a window that thickly it'd have to be a pretty good build up over time, and in my opinion wouldn't make such a clear impression, unless the bird that hit it was wet (which would possibly explain the navigational problems).

Either way I don't really care. That's just what I think.