The Imperial Shag, Phalacrocorax atriceps, is a black and white cormorant native to many subantarctic islands, the Antarctic Peninsula and southern South America, primarily in rocky coastal regions, but locally also at large inland lakes. It is sometimes placed in the genus Leucocarbo instead. It is also known as the Blue-eyed Shag and by many other names, and is one of a larger group of cormorants called blue-eyed shags. The taxonomy is very complex, and several subspecies are often considered separate species instead.
The taxonomy is very complex and species-limits within this group remain unresolved. The following are usually considered part of this group:
- Imperial Shag, Phalacrocorax (atriceps) atriceps, from coastal southern Chile and Argentina.
- King Cormorant/White-bellied Shag, Phalacrocorax (atriceps) albiventer*, from the Falkland Islands, and locally in southern Argentina and Chile.
- Antarctic Shag, Phalacrocorax (atriceps) bransfieldensis, from the Antarctic Peninsula and the South Shetland Islands.
- South Georgia Shag, Phalacrocorax (atriceps) georgianus, from the South Georgia and the South Sandwich Islands, and South Orkney Islands.
- Heard Shag, Phalacrocorax (atriceps) nivalis, from Heard Island.
- Crozet Shag, Phalacrocorax (atriceps) melanogenis, from the Crozet and Prince Edward Islands.
- Macquarie Shag, Phalacrocorax (atriceps) purpurascens, from Macquarie Island.
A white-cheeked P. (a.) atriceps with black-cheeked P. (a.) albiventer on either side. Beagle Channel, Argentina
* The validity of albiventer is questionable, and some recent authorties consider it only a black-cheeked morph of atriceps (sensu stricto). This black-cheeked type also occurs together with "normal" white-cheeked atriceps at some localities in southern mainland South America, especially on the Atlantic side of the continent. There are no known behavioral isolating mechanism between the two and hybrids do occur.
While some authorities accept all of the above "? except albiventer "? as separate species, others consider all as subspecies of a single species (as done in this article). Alternatively, some recognize two species, the white-cheeked P. atriceps (with subspecies bransfieldensis, nivalis and georgianus) and the black-cheeked P. albiventer (with subspecies melanogenis and purpurascens), or it has been suggested that three species should be recognized, P. atriceps (incl. albiventer), P. georgianus (with subspecies bransfieldensis and nivalis), and P. melanogenis (with subspecies purpurascens and possibly verrucosus, though the latter is relatively distinctive, and most consider it a separate species, the Kerguelen Shag).
An immature P. (a.) albiventer in Patagonia, Argentina
The Imperial Shag has a total length of 70-78 cm (28-31 in) and weighs 1.8-3.5 kg (4-8 lbs), with males averaging larger than females. It is endowed with glossy black feathers covering most of its body, with a white belly and neck. It possesses a distinctive ring of blue skin around its eyes, an orange-yellow nasal knob, pinkish legs and feet, and an erectile black crest. During the non-breeding season, adults lack the crest, have a duller facial area, and less/no white to the back/wings.
The subspecies vary primarily in the amount of white on the cheeks/ear-coverts, wing-coverts and back. Most taxa have white cheeks and ear-coverts, but these are black in albiventer, purpurascens and melanogenis. Chicks are uniform brownish, and immatures are brownish and white (instead of black and white), have dull facial skin, and lack the orange-yellow nasal knob and blue eye-ring.
A large colony of P. (a.) albiventer at the Beagle Channel, Argentina. Notice the numerous all-brownish chicks.
This is a colonial, monogamous species. The colonies are usually relatively small, but some consist of hundreds of pairs and are often shared with other seabirds such as Rock Shags, Southern Rockhopper Penguins and Black-browed Albatrosses. The up to 5 eggs (usually 2-3) are placed in a nest made of seaweed and grass, and cemented together with mud and excrements. The eggs usually hatch in about five weeks, and are brooded by both parents. Many chicks and eggs are lost to predators such as skuas and sheathbills.
The diet of this species consists of small benthic fish, crustaceans, polychaetes, gastropods and octopuses. The South American populations primarily feed on fish, especially Argentine anchoita, while P. (a) nivalis primarily feeds on fish and polychaetes. Mean diving dept for the South American populations is almost 25 m (82 ft), while mean for P. (a) nivalis is 5 m (16 ft) and maximum 60 m (197 ft). Most feeding takes place in inshore regions, but at least some populations (e.g. the South American populations) will travel some distance from the shore to fish.
Overall this species is not considered threatened and consequently listed as Least Concern by BirdLife International and IUCN. Most subspecies are relatively common with estimates of 10.000+ pairs of each, the exceptions being P. (a.) nivalis with approximately 1.000 pairs and P. (a.) purpurascens with approximately 760 pairs (has perhaps declined slightly from this figure in recent years). If these are considered separate species, it is likely one "? or both "? would qualify for a threatened status, and they are considered Vulnerable by DEWHA. Due to their small ranges and relatively small populations, they are highly susceptible to pollution and climate changes, and chance events such as storms. Deaths due to strikes with radio masts have been recorded in both, and are quite common in P. (a.) purpurascens. Introduced predators potentially also present a serious threat, though none are currently present on Heard Island and cats have been eradicated from Macquarie Island, leaving "only" rats, which, however, have been observed at nests of P. (a.) purpurascens.