1w Thayer's Gull??

Conclusion can't be put out because there is no photograph from side.

Smaller than 2w Slaty-backed Gull.

Note the bill size with 2w Slaty-backed Gull.

This bird is small type Slaty-backed Gull or Thayer's Gull.
But conclusion cannot be put out from these photographs.

From Mori Hokkaido Japan. December 29.