is the answer…
…apparently to the Mystery bird. Paired with a normal male Gadwall, this female has been present at Slimbridge (a wild bird, not part of the collection) for a number of years. In recent years it has been progressively developing male-like characters in its plumage with each successive moult. It’s still paired to the male- wonder what he thinks of it all?!
P.S. if it helps I thought it might be some kind of Gadwall hybrid when I first saw it (thanks to reserve warden, James Lees for the info)
Reduced function of sex organs in females and development of male-like characters in older females has been long documented. I don’t know much about it, more discussion on the subject here: http://10000birds.com/intersex-mallard-vancouver.htm
The Mystery Bird:
Her bill blackened, head pattern changing, male-like breast pattern, scapulars and blackened rear end. Oddly the flanks are more like a bright version of the female plumage. Tried to see/ photograph the wing pattern for differences, but failed (poor light).