Was going to trash this one as lacking any merit - but then noticed the eye
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It looks like you've caught this Crow while it was blinking. Birds have a third eyelid, called the nictitating membrane, which closes horizontally across the eye. This is usually transparent, but can appear opaque in photographs.
So that's it. Should have thought of it myself. Thanks.
Lat/Lng: 54.7, -1.3
OS grid ref: NZ4836