The imperial shag (Leucocarbo atriceps) is a black and white cormorant native to southern South America, primarily in rocky coastal regions, but locally also at large inland lakes. Some taxonomic authorities, including the International Ornithologists' Union, place it in the genus Leucocarbo, others in the genus Phalacrocorax. It is also known as the blue-eyed shag, blue-eyed cormorant 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 former subspecies are often considered separate species.
The taxonomy is very complex and species-limits within this group remain unresolved. Some taxonomic authorities, including the International Ornithologists' Union, split the group into the species listed below. Others consider the all or part of the group conspecific. The following are considered part of this group:
- Imperial shag, Leucocarbo (atriceps) atriceps, from coastal southern Chile and Argentina.
- Antarctic shag, Leucocarbo (atriceps) bransfieldensis, from the Antarctic Peninsula and the South Shetland Islands.
- South Georgia shag, Leucocarbo (atriceps) georgianus, from the South Georgia and the South Sandwich Islands, and South Orkney Islands.
- Heard Island shag, Leucocarbo (atriceps) nivalis, from Heard Island.
- Crozet shag, Leucocarbo (atriceps) melanogenis, from the Crozet and Prince Edward Islands.
- Macquarie shag, Leucocarbo (atriceps) purpurascens, from Macquarie Island.
While some authorities consider 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 L. atriceps (with subspecies bransfieldensis, nivalis and georgianus) and the black-cheeked L. albiventer (with subspecies melanogenis and purpurascens), or it has been suggested that three species should be recognized: L. atriceps (including albiventer), L. georgianus (with subspecies bransfieldensis and nivalis), and L. melanogenis (with subspecies purpurascens and possibly verrucosus, though the latter is relatively distinctive, and most consider it a separate species, the Kerguelen shag).
The imperial shag has a total length of 70–79 cm (28–31 in) and weighs 1.8–3.5 kg (4.0–7.7 lb), with males usually 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. It has a serrated bill used for catching fish.
The group varies 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.
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. Up to five eggs (usually two or three) are placed in a nest made of seaweed and grass, and cemented together with mud and excrement. 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. They primarily feed on fish, especially Argentine anchoita,. Mean diving depth is almost 25 m (82 ft),. Most feeding takes place in inshore regions, but at least some populations will travel some distance from the shore to fish.
Overall this species is not considered threatened and is consequently listed as Least Concern by BirdLife International and IUCN. Most subspecies are relatively common with estimates of over 10,000 pairs of each
- The validity of albiventer is questionable, and some recent authorities consider it only a black-cheeked morph of atriceps (sensu stricto). This black-cheeked type occurs together with "normal" white-cheeked atriceps at some localities in southern mainland South America. There are no known behavioral isolating mechanism between the two and hybrids do occur.
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- Department of the Environment, Water, Heritage and the Arts (2009). Heard Island Cormorant. Archived 2009-09-12 at the Wayback Machine Heard Island & McDonald Island. Accessed 2009-01-21.
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