Dark-bellied Brent Geese, recent sightings

Interestingly in the last few weeks, there have been quite a few records of dark-bellied Brent Geese (eg Killough, Dundrum and Kinnegar). This seems strange as there are now only very small numbers of light-bellied Brent Geese accompanying these birds. Our light-bellied Brent are heading to Svalbard, Greenland and north-east Canada and nearly all have departed. However where are the dark-bellied birds going? Their breeding range is northern and western Siberia so why are they turning up on the western extremities of Europe at this time of year? Thanks to those who have highlighted this movement. Any feedback or further sightings are welcome.

Dark-bellied Brent Goose with Light-bellied Brent Goose, Kinnegar - Ronald Surgenor:

 

 Dark-bellied Brent Goose, Dundrum, David Nixon: