A Carrion Crow was feeding at the road side. True to form and notwithstanding the severity of the weather, it was instantly spooked by the sight of a human showing any interest in itself, so it hurried off. As it flew it revealed that it was one of those crows that have white patches of feathering in the wings.
This form of plumage occurs regularly in Carrion Crows and birds like these feature regularly in on-line identification forums such as Bird Form. In fact, by strange coincidence one appeared there today (referring to the American Crow, but the plumage aberration is one and the same). Admittedly it can be quite an exciting plumage variation, especially when it forms a full double white bar across all of the flight feathers. I saw a double white wing-barred crow when I was at school in the 1980s and sincerely thought I had stumbled upon something very rare indeed... 
Usually this plumage in crows is described as leucism, a term that is used widely for plumage lacking in, or reduced in, pigmentation. The Oystercatcher on the right (photographed at Musselburgh Lagoons, East Lothian, 26 May 2009), for example, shows much reduction in its melanin pigments with the black feathers replaced by a mixture of white and fawn plumage.Melanin adds to the strength of the feather structure. As a result black feathers are more resistant to wear (and maybe bacterial degradation) compared to white feathers. This additional strength afforded by the dark melanin pigments possibly explains the regular occurence of black wingtips in many different species of birds. Conversely, as the less resistant pale fringes of many feathers are worn away, the appearance of a bird's plumage may change without the need to moult. Certainly, the paler patches of these odd Carrion Crow's plumage often appear to be suffering heavily from the effects of abrasion.
However, I do not think that these areas of feather de-pigmentation have the same origin as either the overall leucism shown in the Oystercatcher above or in the individual white feathers that show up regularly in species sucha s the Blackbird. These aberrations are likely to be due to the switching on or off of particular genes in the cells responsible for feather development. Instead, in the crows it is likely that this plumage aberration is a symptom of a dietary deficiency. The scarcity or absence of a particular nutrient, or precursor substrate, is preventing sufficient successful completion of the metabolic pathway generating melanin. The tail of the Carrion Crow pictured above certainly appears to show growth bars that indicate some variability in health or dietary status during the period those tail feathers were grown (given the uniformity of the bar, presumably this is a 1CY bird that grew all of these feathers at the same time?).
So what evidence is there that this plumage aberration may be the result of a dietary deficiency:
Calcium deficiency is cited as being the cause of this plumage by at least some bird rescue centres (and here). Crows that are severely affected by this condition grow normal plumage at their next moult once given an appropriate diet. Whether there is evidence for the deficiency being due to a lack of calcium is unclear, but this appears to be strong evidence that this plumage aberration has an environmental cause and is not solely under genetic control.
Folic acid deficiency has been found to cause feather de-pigmentation in domestic chickens. The feather producing tissues originate from neural crest cells and folic acid is known to affect neural crest cell differentiation. Of course, crows are distantly related to chickens, so some metabolic pathways may be different, but there is a very strong link between this plumage aberration and environment.
In addition, it is interesting that viral infection can lead to this symmetrical feather depigmentation:
Circovirus infection evidence has been found in Australian Ravens with this these white patches their wings. In this case the feathers also had thickened sheaths which did not allow the correct development and opening out of the feathers. It is possible that our crows are also affected by a similar circovirus and that this is an underlying cause of the white patches we see in our birds. Again, this evidence from Australia strongly supports the idea that an environmental agent can be a key factor in the development of this aberrant plumage.
It is worth noting that there is no evidence that this plumage aberration is a solely inherited characteristic. Instead, it seems reasonable to conclude that it is likely that an environmental factor (e.g. diet or viral agent) is a determining factor influencing whether the correct melanin development occurs in the wing feathers of crows. It may be that feather de-pigmentation is a better term than leucism for this type of aberration, though, of course, it is unlikely to catch on...
