I have included this composite photo to show the subtle differences between the fabalis and rossicus sub species. Disregard the plumage colouration differences as these are caused by one bird being in full sun and the other being photographed on a cloudy day.
The field guides describe differences in structure, mainly in the neck length, and in the amount of orange colouration in the bill. The neck length difference is quite subtle and can easily be complicated by the posture of the bird - an alert bird will have a longer neck length than a relaxed one. I do not feel that the bill colouration is a reliable separation as there is a lot of variation in the amount of orange in the bills of the Taiga fabalis birds. This can be seen by reviewing the photos of the fabalis birds in the Fannyside flock.
Some observors consider that the two sub species cannot be reliably separated in the field. This is further compounded by the wary, skittish nature of the birds which makes close observation very difficult.
Any comments would be welcome.