OMG, that would be impossible to treat here (catching the ducks !) - I've only ever found Red Mite in the hen house but no evidence with the ducks. The IR chose not to swim. she only ever dipped her head in water, maybe that's what may have made her a victim of NFmites. I don't know if she was still laying , I can only try to keep taps on those who are laying outdoors...The only other thing I remember recently was that she tried to fly out of the house in the morning and she landed quite hard, maybe she hurt herself internally....it seemed to have come on fairly suddenly. It's all guesswork now but thanks for the suggestions, will keep a close eye on the others.