A Marsh Harrier is, I would guess, more formidable than a crow. However, when set about by many during flight, the Harrier is at a disadvantage. This is often seen with Common Buzzards and Red Kites too when Crows, Rooks and Ravens gang up to see the birds of prey away from their nesting area.
I think the Crows like tall trees overlooking fields and this Marsh Harrier came across the flooded fenland, over the dike and bird hides and into the inner fields where a line of trees were; the bird of prey set off a panic among the carrion birds.
Carole and I were walking between bird hides. The one we were at was close to a shooting group who were blasting away at water fowl and using whistles to direct the gun dogs. We decided to go further along the dike and away from the shooting group.
It was upon the muddy bridal path that we see the Marsh Harrier incident. Again, I was clicking away at distance and most shots are too far, but I've managed to salvage a few to give the reader an idea of what happened.
Even I could tell the Marsh Harrier. It has a distinct flight signature and it looked menacingly graceful and elegant as it swirled about by the tree tops. The serene incident did not last long as the carrion began to squawk and gather to defend their trees and airspace.
On the final shot, the Marsh Harrier is in the bottom right hand corner with a Crow behind and others in pursuit. It was seen off across the flooded Fen, but did return again later.