The Girl with the Broken Wing

The twins are in bed when it happens. They hear a sound like a wet cabbage hitting the wall, then a clattering on the roof tiles, followed a little later by a tapping at the window. It's a girl with a broken wing - and she's come to stay. Who is she? Where has she come from? And more importantly, what does she want? The twins have no idea. Perhaps she's their guardian angel? But would an angel have filthy feet and snore?