wait--I have a physics question.
Things fall in a downward direction, due to gravity, yes?
Of course, they may start in a lateral/horizontal direction, depending on the force with which they begin their journey (a la water squirting from a hose).
Windows are generally vertical, yes? I guess there might be some deviation, if a house is old, not level, etc.
But I'm wondering how likely it would be that poop from a butterfly--whose legs lift its body away from the surface it is standing on--would exit with enough force that it would land on a window pane. On the sill, that makes sense. On the glass itself?
OP, can you clarify--is the butterfly poop on the glass, or on the sill?