So what is it, if not a first-winter male? I missed a clue from the start that this wasn't a first-winter, I think. The wing-coverts have - and have had since I first saw it - clear whitish fringes. I believe first-winters should have browner fringes, less clear-cut than these. It can't be an adult male with these non-white wing-coverts, so it must be an adult female. But it has male-like grey feathers coming through on the scapulars and flanks, and the head is more solid reddish brown than I would expect on a female. I think there's even a hint of the cream crown - just faint, but looking rather like it can on first-winter males where it's just beginning to come through. In fact the pattern of grey male-like feathers does make it look a lot like a first-winter male - it wasn't completely stupid of me to jump to that conclusion when I first saw it! Here's a first-winter male (already with white wing-coverts) that looks fairly similar:
So the Salthouse bird is an adult female with some male features - an intersex, presumably. But I had the impression that intersex birds develop over time, becoming more male-like as time goes on. This bird is no more male-like now than it was four winters ago, so that seems surprising if it is an intersex - but what other explanation can there be? Do some intersex birds only ever go so far towards male-like plumage?
Closer inspection reveals some other features that don't seem quite normal on male or female Wigeon. The extent of dark barring on the breast is greater than I would expect, though this is normally variable on Wigoon. There's also a bit of dark barring on the fore-flanks. Maybe these are features thrown up by intersex Wigeons? There is precedent for that happening with intersex ducks - intersex Pintail can show dark barring on the flanks, unlike any normal plumage of Pintail (eclipse male comes closest).
And the white on the head? Is it too big a coincidence that one bird should show two different abnormal conditions (leucism as well as intersex)? Probably not - intersex birds seem to be more prevalent among birds already suffering from some kind of abnormality, e.g. in hybrids or selectively-bred mutants.
If this is a female developing male characteristics, are we right in calling it intersex? Or are there other conditions that lead to the same phenotypic outcome?