I would just leave it. Not sure why on earth Mama Bird laid it there when she has a nest but who knows. Maybe someone was in the process of stealing it and dropped it on your window sill? One thing I have learned with having my birds around is that they never seem to notice missing eggs. It breaks my heart a little bit every time I have to take one out of the cage but they just go on about their business like there was never anything there to begin with.