About this bird: It does not look like a 100% sibiricus. Is it a cross? A cross with a homeyeri/leucopetrus “influence”? Or more probably just a rare variant within the sibiricus group?
There is another individual with similar characteristics in the sibiricus collection of the Moscow Museum.
The bird was sampled and showed a sequence of 634 bp of the mtDNA gene CO1. Below is the “Fast Minimum Evolution” “Blast” tree created after comparing the Shemya birds CO1 sequence to others in GenBank. The bird (highlighted in yellow, “Query_26821”) appears to group with other borealis, but the rough tree topology is similar to that found with other mtDNA genes, by for example Olson et al. (2010; Fig. 2). Based on that, it seems more than likely to represent simply variation within sibiricus or bianchii.
Photographs © Mike Schwitters