Please not how the head shape changes in several of the Thayer’s from Japan depending their posture and behaviour.
I do not think the head of the Danish bird is neither that abnormally flat, nor its bill unusually big. Perhaps my assessment of shape is severely lacking, but I would not say that the head of the Danish bird looks neither flatter than any of these six Japanese examples, nor that its bill looks bigger. For anyone who wants to check, the five examples photographed by Masashi Shinjo in Choshi can be found in their proper context here, respectively: here, here, here, here and here.
|Thayer’s Gull: upper row: 1-6: 4.2.2012, Hvide Sande Harbour, Denmark, Kent Olsen; lower row: 7-12: six different adult or subadult Thayer's Gulls from Japan, Peter Adriaens & Masashi Shinjo.|