odd Marsh Harrier tail

surveying harriers this afternoon came across this male; clearly an old adult male from the upperwings and the underpart pattern it oddly has a dark brown central tail with just the outers being pale grey; it was very obvious when the tail was spread but as I only managed a few images as it passed me the full extent of the colour is not clear; it could I suppose be staining but as it is only on the central tail feathers it seem odd - needs further study