ad. winter taimyrensis (taimyrensis/vegae intergrades)
Sakawagawa river, Odawara, Kanagawa, Japan, April 2nd 2001.

Most taimyrensis (or taimyrensis/vegae intergrades)
remain winter plumage that has streaking on hindneck in spite of early April.

Taimyrensis seems intermediate form between vegae (birulai) and heuglini by nature.
So It is not easy to say what is "pure taimyrensis".
Mantle shade is often very close (or almost same) to vegae.
But, some darker examples have similar tone to black-tailed gull or slaty-backed gull 
(therefore similar to heuglini ).

Typical darker individual was not observed on that day.
But it is often not easy to assess mantle shade because it can vary by condition of sun light.
Each photos capture different (probably) individuals except for 2 photos at the top.

Male-type large individual.

Note the body structure unlike the slaty-backed gull in background. 
(lighter, slimmer impression and longer wing, ) 

This bird has a lot of typical features (especially in structure) of taimyrensis.
But the mirror on P10 is unusually large.

The heavy head streaking and paler mantle may be influence of vegae.
But smaller mirror on p10 and yellowish legs ( not shown here) are taimyrensis-like features.
Such individuals usually seem not simple "yellow-legged vegae" because they 
also show later primary molt than vegae in autumn or mid winter.

Whiter head (though it has slight streaking), head/bill structure, 
and large mirror on P10 seem not typical for taimyrensis.
This resembles "yellow-legged mongolicus" too. 
This type is difficult especially in spring, because primary-molt has finished. 

The mantle shade was only a little darker than vegae, but has typical taimyrensis-impression.
Note the small mirror and  large red bill-spot.

This is the darkest bird in that day.

This has strange wing pattern that show more white on its primaries just like Scandinavian herring gull ( L a.argentatus)

The legs were pale flesh color (but yellower than vegae) and  webs were pinkish.
The mantle shade was similar to vegae. This also is similar to some mongolicus and difficult.