[3] The adult's upperparts, tail, and thighs are metallic greenish black; the underbody to the throat is white; and the wing linings are brown.

The head and the back of the neck are deep blue or purple, except that a black cap reaches below the eye to the chin and ear coverts.

Breeding adults have a little black erectile crest on the forehead, yellow or orange caruncles (large warts) above the base of the bill, and a bright blue ring around the eye.

After the breeding season, the plumage fades, the eye-ring becomes lead-blue, and the caruncles become smaller and duller-colored.

[3] Kerguelen shags presumably eat mostly fish and such invertebrates as echinoderms, crustaceans, and polychaete worms.

In summer they mostly forage alone, but from May to October they form linear flocks of several hundred, diving and surfacing sequentially down the line.

There is a period of nest-building and displaying in late March and early April—an unusual time for a subantarctic cormorant.

Advertising males display by throwing the head back till the nape touches the tail.