Sorry about the image quality it was very low light.
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I think I'm just just starting to see a common theme to your uploads! Is this purely genetic or the result of mild embryonic damage?
I think the exact cause can vary.
I believe there is some suggestion that dietary deficiency is the most common cause but whether there is a genetic component to that (i.e. a genetic susceptibility to suffering from dietary deficiency) is not clear.
I've often wondered about the cause. From my observations I think that over the years I've seen similar plumages in the roughly the same locations. But that could be genetic or diet related.
James P Deans
This is commonly seen in Carrion Crows in some areas near me, and I know that it has been recorded in crows in other parts of the world (eg. American Crows).
Lat/Lng: 56.1487063, -3.7401282
OS grid ref: NS919964