Brant #2.1 The bird under discussion is the third goose from the right. This bird showed even greater contrast between black neck sock and the brownish wash across the anterior flanks and breast than Brant #1.
Brant #2.2 The break in the neck collar is just visible. The bird was quite aggressive snapping with out-stretched neck at birds that came too close. The neck collar appears more extensive than typical for Atlantic Brant.
Brant #2.3 & Brant #2.4. These two shots clearly show the break at the center of the collar. The collar itself appears quite tall with an extensive lattice of diagonal bars.
Brant #2.5-2.8.The clear break in the neck collar is visible in photo 2.4.
In their seminal 2001 Birding World article, Garner and Millington write:
"Typically the overall appearance of Grey-bellied Brant is close to that of [Atlantic Brant], but with more darkly pigmented underparts. It should be stressed that the belly is not pure grey, but varies from brown to brownish-grey, and this darker area often tends to extend to between the legs or just beyond (about in line with the rear flank feathers). This coloured 'bib' is not as extensive or as dark as on Black Brant, and there is always an obvious contrast between the dingy belly and the lower boarder of the black chest. A key feature for telling identifiable Grey-bellied Brant from [Atlantic Brant] is that, with careful observation, the area is often similar in extent to that on Dark-bellied [nominate] Brent Goose."
Upperparts: Varies from being similar to Atlantic Brant to being darker and more uniform. Black Brant tend to be darker still.
Collar: "Varies from being similar to that of [Atlantic] Brant, to being more obvious and on many birds approaching that of Black Brant in both height and extent. The lower part of the white band can just meet across the front of the neck...... and the white 'rivulets' running up the neck sides can be quite tall."
Citing goose researcher Hugh Boyd, an authority on this population from the nesting grounds, Garner and Millington note that "Grey-bellied seems to be the most variable of all the brent geese; they vary from some being apparently indistinguishable from [Atlantic Brant] through to darker individuals which approach Black Brant in appearance."
M and Millington, R (2001) Grey-bellied Brant and the Dundrum conundrum.
Birding World 14(4): 151-155