On a walk with the kids today, saw the same Carrion Crow showing feather de-pigmentation. It was in a group of five birds, three of which showed at least some signs of this aberration.
When I told the kids that this bird was showing signs of a probable dietary deficiency they looked at me as if I wasn't very bright and responded, "Well, feed it then...". A small handful of our nut and fruit mix later and the crows were winging off to eat or cache their new supplies.
Note that in the third photo part of the de-pigmentation resembles plumage of a Hooded-Carrion Crow hybrid, though it is likely that it is not indicative of any recent Hooded Crow ancestry in this individual but merely a symptom of the presumed dietary deficiency.