I know leucism and other pigment-deficiency conditions can affect birds in different ways, for example making the whole bird very pale, or just making parts of the bird white, but the situation with Wigeon strikes me as being different from most other instances.
Thinking of the birds I've seen that have had patches of pure white in their plumage, this white can appear in different places on different individuals. I've seen lots of Pink-footed Geese showing white patches but the position varies by bird (though typically it includes primaries and bands across the neck or belly). I've seen lots of Blackbirds with white plumage, but haven't noticed any pattern as to which bits will be white. But Wigeon seem different. I've seen half a dozen Wigeon with obvious white patches and each one has had this white patch on the head. The exact size and shape has varied (usually diffuse and sometimes more or less tending towards speckling) but always just behind the eye.
Here are some of them:
female Wigeon, Caerlaverock (Dumfries & Galloway, UK), 29th December 2005
Why is it always the side of the head that is affected - I've never seen a Wigeon with patches of white anywhere else (I have seen generally washed out birds, but that's a different condition affecting all or most of the plumage making it paler than normal, not white).
Brent Geese are the same. Again I've seen birds that are generally washed out, paler than normal all over, but the birds I've seen with abnormal pure white plumage affecting only a small part of the bird have all been similar - they've all had white speckling on the head, albeit varying in extent. Here are a few, but I've far more in addition to these, all similar.
Dark-bellied Brent Goose, Cley (Norfolk, UK), 28th February 2004
Dark-bellied Brent Goose, Cley (Norfolk, UK), 5th January 2008
Dark-bellied Brent Goose, Salthouse (Norfolk, UK), 20th November 2004 (a bird with just a few white flecks)
Dark-bellied Brent Goose, Stiffkey (Norfolk, UK), 31st January 2009
Is this the same with the other forms of Brent Goose? I've only seen it in Dark-bellied but that's probably just because it's the only form I see thousands of every year.
Update January 2015
Yet another Wigeon with white on the head here...
Update December 2015
Seems that the first two birds from Salthouse in February 2014 and December 2012 were in fact both the same bird, as it is back in December 2015, still looking the same. Can't be a first-winter then and I assume it's an intersex female. More on that in this post.