Pied Wagtail

Grey Wagger yesterday and a Pied Wagtail today.
  These are the ones you often see in the roads,dodging among cars.
I believe they do it to pick up grit for their crop.The same as a Budgie does I suppose?
 It was on the flat roof,outside my bedroom window,picking up food that I put out there for the birds.
  It was only about 6-8 feet away.The other side of the window glass I must say.But it was not afraid?