Imperial shag  (Phalacrocorax atriceps)

Classified as Least Concern (LC) on the IUCN Red List (1).

A strong swimmer and a proficient predator of fish, the imperial shag, also known as the imperial cormorant, displays a number of adaptations for underwater hunting. Like other cormorants, its bones are proportionately heavier than other seabirds, which along with a lack of body fat, reduces the bird’s buoyancy on water. The feet are fully webbed and powerful thrusts of the set-back legs propel the streamlined body through the water in pursuit of its prey. It has a long, sinuous neck, sealed nostrils, and a sharply hooked bill, which can grip slippery fish (3). The male and female imperial shag are much alike, with glossy black upperparts, white undersides and pink feet (2). A conspicuous tuft of black feathers stands prominently on the forehead, while a ring of blue skin surrounds the eyes and a pair of bright orange, knob-like warts (known as caruncles) sit above the base of the bill (2) (4) (5). Juveniles differ from adults in their predominantly brown plumage, with a slight greenish tinge to the upperparts, and the lack of caruncles and head crest (4) (5). Although cormorants are largely silent birds, the imperial shag does have a variety of vocalisations, including a rasping bark made by the male during aggressive encounters and a hiss made by the female (4).     

The imperial shag has a wide but discontinuous distribution across the Southern Ocean. It is found around the southern tip of South America (from Tierra del Fuego and Cape Horn northwards to central Chile and Argentina), the Antarctic Peninsula, the Falklands, South Orkney and South Sandwich Islands, South Georgia and Heard Island, with isolated populations on Crozet Island and the Prince Edward Islands in the southern Indian Ocean, and Macquarie Island in the Southwest Pacific (4) (5) (6) (7).

The imperial shag inhabits rocky coasts and islands surrounded by deep, sub-Antarctic waters, although it typically forages around shallower, inshore areas. During the breeding season this sociable bird forms large colonies, often with penguins or albatrosses, to nest on cliffs and rocky islands (2) (4) (6).

Cormorants are sleek, efficient predators which catch their prey after an underwater pursuit, whereby the fully webbed feet propel and steer the streamlined body forwards (2) (3). These birds have an unusually high volume of blood in their bodies, and the large amount of stored oxygen enables them to remain underwater for up to four minutes at a time (3). Owing to this unique adaptation, the imperial shag can dive as deep as 50 metres to catch its fish or crustacean prey under rocks or in kelp beds (4) (5). The inner feathers are waterproofed and insulate against the cold waters; however, the outer feathers become sodden, limiting hunting forays to only two bouts of 30 minutes before the bird must return to land to allow the waterlogged plumage to dry (3). Upon returning to land, the imperial shag assumes a characteristic posture and stands upright with the wings extended outwards, exposing itself to the sun to dry (3) (4).

At sea, the imperial shag may forage individually or in large flocks of thousands of birds, but when breeding, this bird always forms dense colonies, occasionally numbering as many as several hundred thousand birds (2) (4). Breeding pairs regularly change nesting sites between seasons, but competition for the best location is always fierce and breeding birds will engage in aggressive territorial behaviour. Typically this rarely goes beyond threatening postures, but on occasions when it does, fighting may ensue, with bill-grappling or birds seizing the opponent by the neck and wings (3) (4). The male bird gathers vegetation, mud and guano to build a simple cup-shaped nest in which a clutch of normally three eggs is laid (2) (4). Both the male and female take turns in incubating the eggs for around 28 days (3) (4). The chicks hatch naked and helpless, but grow quickly on an energy-rich fish diet and will fledge after around 75 to 80 days (4). After leaving the nest, the young birds may already be capable of flying, but if not, they gather together at sea in small crèches until they are strong enough to become fully independent (3).

As the imperial shag largely inhabits remote, inhospitable areas and forages around inshore areas, away from deep-sea commercial fisheries, it rarely interacts with humans (5) (6). Consequently, this species has never been intensively exploited and lacks any significant threats. However, some smaller, isolated populations, such as those on Heard and Macquarie Islands, are vulnerable to extreme weather events destroying nesting habitat, killing chicks and limiting foraging opportunities. Climate change could also potentially threaten the imperial shag by reducing its prey abundance, while marine pollution and debris could kill some birds (5) (7).  

Although not the target of any known specific conservation measures, the imperial shag has likely benefited from efforts to protect and restore natural seabird habitats and remove invasive predators, such as those being undertaken on the Falkland Islands and South Georgia (8) (9) (10).

ARKive is supported by OTEP, a joint programme of funding from the UK FCO and DFID which provides support to address priority environmental issues in the Overseas Territories, and Defra

To find out about conservation projects in the Falkland Islands and South Georgia, see:

• FalklandsConservation:
  http://www.falklandsconservation.com/
• The South Georgia Heritage Trust:
  http://www.sght.org/
• Procter, D. and Fleming, L.V. (1999) Biodiversity: the UK Overseas Territories. Joint Nature Conservation Committee, Peterborough, UK. Available at:
  http://www.jncc.gov.uk/page-3045%23download

For more information on this and other bird species please see:

• BirdLife International:
  http://www.birdlife.org