Greetings All
The dark coloration of the Houts Res bird was more extensive than should be seen on any Atlantic Brant, at least in the USA/Canada wintering grounds. Additionally, on close inspection (see my google site) the Houts Lake bird has a complete necklace, which should not be present in Atlantic Brant.
Now, some Gray-bellied Brant are nearly white beneath and are indistinguishable from Atlantic Brant (birds ID'd by genes and breeding location). Similarly, the Atlantic Brant that winter in Ireland and nearby locales breed in the Eastern High Arctic, and they are different from those wintering in the USA. They are more likely to have a tad of gray underneath, possibly from some genes infused by Dark-bellied Brent Goose (much like the gray bellies of Gray-bellied Brant may well arise from interbreeding with the occasional Black Brant).
Cheers
Steve Mlodinow
Longmont