Better adapted to swimming than flight, the rock shag has powerful webbed feet, dense bones and very little body fat, making it an astonishingly agile predator (4)(5). In both sexes, the black feathers on the head and neck are tinged with a metallic blue or green sheen, which becomes purple and flecked with white filoplumes on the upperparts. The upper back and wings are dark, with a deep green gloss and black markings on the feathers, while the lower back is blue-black with many white filoplumes. The underparts and throat are white. Contrasting with its predominantly black and white plumage, the rock shag has distinctive brick-red eyes, with a bright red orbital ring (the ring of skin around the eyes) and a red face. In breeding plumage, the rock shag has a crest of black feathers above the forehead, and distinct white patches over the ear. Juvenile rock shags have brown feathers, which are dark on the head, neck, back and wings and paler elsewhere, with mottled black-brown on the lower breast and underparts (6)(7).
A skilled underwater hunter, the rock shag spends over a third of the day foraging alone at sea, usually within five kilometres of the colony (8)(9). The rock shag generally dives to depths of no more than 15 metres and favours areas close to kelp fields with a gravelly sand bottom, where it searches the sea bed for small benthic fish, crustaceans, cephalopods and polychaete worms. On a single foraging trip, the rock shag may spend as much as 92 percent of its time diving, sometimes making more than 100 dives in a day (3)(6)(8)(9)(11). Like other cormorants and shags, it has a relatively large volume of blood in proportion to its body weight, enabling it to store enough oxygen to remain submerged for several minutes if necessary (4).
The rock shag lives in small, isolated colonies along the coast and, somewhat unusually for a seabird, most individuals remain within the colony throughout the year, often occupying the same nest site even between breeding seasons (10). The male rock shag attracts the female with a ritualised courtship display, throwing the head back and performing a series of ‘wing flicks’ and vocalisations as the female approaches. Breeding occurs between October and December, depending on the location, and a clutch of between two and five (most often three) eggs are laid by the female in a cup-shaped nest built from seaweed, tussock grass and leaves, which is cemented together by mud and guano(2)(6)(7)(11).
Breeding colonies of the rock shag are widely distributed along the southern coasts of Argentina and Chile, including Tierra del Fuego and throughout the Falkland Islands. Most individuals tend to remain close to the breeding colonies during the non-breeding season, although some birds have been known to overwinter as vagrants as far north as Uruguay (7)(8)(9)(10).
Restricted primarily to coastal areas, the rock shag is often found along rocky coastlines in channels and sheltered bays, while in the Falklands it is also found to use harbours, estuaries and inland waters. The rock shag typically nests on cliff ledges and on top of steep-sided rocks or islets, as well as in gulleys, caverns and occasionally on exposed shipwrecks and jetties (2)(5)(7)(11).
Because the rock shag breeds mainly on steep, inaccessible cliffs, the species has so far faced relatively few threats and the population is not globally threatened (2). However, increasing human disturbance is a growing concern, as the coastal waters of the South Atlantic are becoming increasingly polluted by oil and rubbish and expanding ecotourism industries bring rising numbers of tourists to seabird colonies. The effects of rising levels of human interaction may include a reduction in breeding success, particularly as the rock shag is prone to abandoning the nest, leaving the young open to predation (12)(13).
Although the rock shag is not the target of any specific conservation measures, it is likely to have benefited from efforts to protect and restore natural seabird habitats and remove invasive predators, such as those being undertaken on the Falkland Islands (13).
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The lowermost region of a marine habitat, the bottom.
From the Greek for ‘head-foot’, a class of molluscs that occur only in marine habitats. All species have grasping tentacles, and either an internal or external shell. Includes nautiloids, cuttlefish, squids, octopuses, and extinct ammonites and belemnites.
Diverse group of arthropods (a phylum of animals with jointed limbs and a hard chitinous exoskeleton) characterised by the possession of two pairs of antennae, one pair of mandibles (parts of the mouthparts used for handling and processing food) and two pairs of maxillae (appendages used in eating, which are located behind the mandibles). Includes crabs, lobsters, shrimps, slaters, woodlice and barnacles.
In birds, simple, hair-like structures, with several soft barbs at the tip, which grow in circles around the base of other feathers.
Accumulated droppings found where large colonies of animals such as seals, bats or birds occur; it is rich in plant nutrients.
Polychaeta means ‘many bristled’; this class of worms are segmented and bear many ‘chaetae’ (bristles).
Yorio, P., Frere, E., Gandini, P., and Schiavini, A. (2001) Tourism and recreation at seabird breeding sites in Patagonia, Argentina: current concerns and future prospects. Bird Conservation International, 11: 231-245.
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