[3] The brant was formally described in 1758 by the Swedish naturalist Carl Linnaeus in the tenth edition of his Systema Naturae under the binomial name Anas bernicla.

The body of the dark-bellied nominate subspecies B. b. bernicla is fairly uniformly dark grey-brown all over, the flanks and belly not significantly paler than the back.

The total population is about 250,000, with the main population breeding in northeastern Canada and wintering along the Atlantic coast of the U.S. from Maine to Georgia,[11] and two smaller populations, one breeding in Franz Josef Land, Svalbard, and northeastern Greenland and wintering in Denmark, northeast England, and Scotland, and the other breeding in the far-northeastern Canadian islands and wintering in Ireland, southwest England, and in a small but significant area, le havre de Regnéville, centered on the Sienne Estuary in Manche (Northern France).

A fourth form (sometimes known as the 'gray-bellied brant) has been proposed, although no formal subspecies description has been made as yet, for a population of birds breeding in central Arctic Canada (mainly Melville Island), and wintering on Puget Sound on the American west coast around the U.S./Canada border.

[20] Individual birds when wintering generally remain in loose family-groups, together with others of the same sub-species, but there is overlap in some areas (for example Western Europe, see above); and this is also true in the breeding colonies.

The brant goose was strictly coastal bird in winter, rarely leaving tidal estuaries, where it feeds on eelgrass (Zostera marina)[21] and the seaweed, sea lettuce (Ulva).

[11][22] On the east coast of North America, the inclusion of sea lettuce is a recent change to their diet, brought about by a blight on eelgrass in 1931.

[24] In recent decades, it has started using agricultural land a short distance inland, feeding extensively on grass and winter-sown cereals.

Food resource pressure may also be important in forcing this change, as the world population increased over 10-fold to 400,000-500,000 by the mid-1980s, possibly reaching the carrying capacity of the estuaries.