Revised opinion this is a MALE intruder that fights the female of the existing pair

edit 19th august An established pairing is challenged by a stronger female (I originally thought now sure it was a male) newcomer, resulting in a full on confrontation between the two female birds. The punchline is that the winner instead of staying to start a family disappeared the next day not to be seen again until midsummer when she (he) returns to cause (see Fiesty Chick vid. )