Taimyr Gull

Taimyr Gull Larus taimyrensis in the North Sea

During summer tens of thousands non-breeding and or immature gulls stay in the North Sea. From the end of July onwards they leave towards the wintering grounds. A substantial number of these gulls began moulting part of their plumage including part of the primaries and secondaries  before they leave the North Sea. On 4th September 2020 a 3CY Taimyr Gull Larus taimyrensis actively moulting primaries and secondaries was briefly seen on the Brouwersdam, Province Zuid-Holland in the SW Netherlands.

see also;

       https://radioactiverobins.com/archive/gulls/taimyrgull/taimyrgull.htm