The imperial shag (Phalacrocorax atriceps) is a black and white cormorant native to southern South America, primarily in rocky coastal regions, but locally also at large inland lakes. It is sometimes placed in the genus Leucocarbo. 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. The following are considered part of this group:
- Imperial shag, Phalacrocorax (atriceps) atriceps, from coastal southern Chile and Argentina.
- King cormorant/white-bellied shag, Phalacrocorax (atriceps) albiventer,[NB 1] 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 Island 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.
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 (including 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).
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 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. 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.
- BirdLife International (2012). "Phalacrocorax atriceps". IUCN Red List of Threatened Species. Version 2013.2. International Union for Conservation of Nature. Retrieved 26 November 2013.
- Nelson, J. Bryan (2006), Pelicans, Cormorants, and Their Relatives: The Pelecaniformes, Oxford University Press, U.S.A., pp. 489–493, Plate 8, ISBN 978-0-19-857727-0
- Shirihai, H. (2002). The complete guide to Antarctic Wildlife. Alula Press. ISBN 0-691-11414-5
- Jaramillo, A., Burker, P., & Beadle, D. (2003). Birds of Chile. Christopher Helm. ISBN 0-7136-4688-8
- Rasmussen, P. C. (1991). Relationship between coastal South American King and Blue-eyed Shags. Condor 93: 825-839.
- Siegel-Causey, D. (1986). The courtship behavior and mixed-species pairing of King and Imperial Blue-eyed Shags (Phalacrocorax albiventer and P. atriceps). Wilson Bulletin 98: 571-580.
- Orta, J. (1992). Phalacrocoracidae (Cormorants) in: del Hoyo, J., Elliott, A., & Sargatal, J. eds. (1992). Handbook of the Birds of the World. Vol. 1. Ostrich to Ducks. Lynx Edicions, Barcelona. ISBN 84-87334-10-5
- Clements, J. F. (2007). The Clements Checklist of Birds of the World. 6th edition. Christopher Helm. ISBN 978-0-7136-8695-1
- Marchant, S.; Higgins, P. J. (2002), HANZAB species list (pdf), Birds Australia, retrieved 2007-10-11
- Dickinson, E. C. eds. (2003). Howard and Moore Complete Checklist of the Birds of the World. 3d edition. Christopher Helm. ISBN 0-7136-6536-X
- Mary, Trewby (2002), Antarctica: an encyclopedia from Abbot Ice Shelf to zooplankton, Auckland, New Zealand: Firefly Books Ltd., p. 38, ISBN 1-55297-590-8
- Punta, G., Yorio P., and Herrera, G. (2003). Temporal patterns in the diet and food partitioning in imperial cormorants (Phalacrocorax atriceps) and rock shags (P. magellanicus) breeding at Bahía Bustamante, Argentina. Wilson Bulletin 115(3): 307-315
- Department of the Environment, Water, Heritage and the Arts (2009). Heard Island Cormorant. Heard Island & McDonald Island. Accessed 2009-01-21.
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