We keep 11 ducks and a drake. Six of the ducks are Cayuga all hatched last summer but came from three different sources as a trio, a pair and a single one hatched by one of our chickens. Over the last few weeks the feather condition of these young ducks has deteriorated. The feathers on their backs are looking brown and rough instead of the usual shinning black/green. The ducks seem well but look awful. I have looked for information on what might have caused this and there seem to be a range of possibilities, infection, mites etc. However, what is strange is that none of the older ducks or the drake have been affected at all. Even a couple of quite elderly ducks are fine. Can anyone suggest what the cause might be and what I need to do ?