crows regularly harrassing magpies whilst they are constructing their nest

Over the past three weeks or so I have been watching a pair of magpies building their nest in a poplar tree near my garden.  However, on a number of occasions each day, two crows move in (even going into the nest) and remain there for several minutes.  There isn't a lot a squabbling, but the magpies slowly move back in and the crows fly away.  I would like to know:  Have others witnessed such events?  Who may win in the end?  If the magpies mate and then have their nest taken off them, what would happen?

I look forward to hearing any comments on this subject.