Cape gannets are preyed on by great white pelicans and humans. They are large birds that are capable of deterring predators at nests and their habit of nesting colonially on offshore islands help to reduce predation risk on eggs and young.
Known Predators:
Cape gannets are large seabirds, 85 to 94 cm in length and about 2600 g. Like other gannets and boobies (Sulidae), they have a characteristic sleek, but robust body, with strong, webbed feet and a long, robust bill. They have white plumage on most of the body, with yellow on the head, chin, and neck and black primary and secondary wing feathers and tail. About 10% of individuals have white feathers in the tail as well. They have a dark gular stripe on their throat, which is longer than those found in other Morus species. Their legs, feet, and webbing are black and their bills are pale yellow with black markings and black skin around the eyes. Juvenile Cape gannets have uniformly brown plumage, gradually becoming white as they mature. They can be confused with masked boobies (Sula dactylatra), which have white heads, northern gannets (Morus bassanus), which have white tails and secondary feathers, and Australasian gannets (Morus serrator), which have only the central tail feathers black. Sexual dimorphism and subspecies are not described.
Average mass: 2600 g.
Range length: 85 to 4 cm.
Other Physical Features: endothermic ; homoiothermic; bilateral symmetry
Sexual Dimorphism: sexes alike
Longevity in Cape gannets is not reported but they are long-lived, as are other sulids. Natural adult mortality is generally low, less than 10%, and adults live from 10 to 20 years, or as high as over 40 years old.
Cape gannets are found in coastal and offshore waters, usually less than 120 km from land. They forage mainly over continental shelf areas but are also sometimes seen in pelagic waters. They breed in open areas on flat or gently sloping, offshore islands.
Habitat Regions: temperate ; tropical ; saltwater or marine
Aquatic Biomes: pelagic ; coastal
Cape gannets were previously known as Sula capensis and Dysporus capensis.
Cape gannets are typically silent except at breeding colonies, where they make a raucous "arrah arrah" call. Boobies and gannets are social birds and use a variety of calls and visual displays, but these have not been described in Morus capensis.
Communication Channels: visual ; acoustic
Perception Channels: visual ; tactile ; acoustic ; chemical
Cape gannets are considered vulnerable because of their limited breeding range, population declines associated with persecution and exploitation, and continued declines in habitat quality and foraging near breeding colonies as a result of pollution and overfishing. The collapse of the Namibian sardine fishery has seriously impacted Namibian populations (Engraulis encrasicolus). They are also sometimes entangled in fishing gear when they accompany commercial fishing operations to take advantage of discards and aggregations of fish in nets. They are sometimes captured on long lines used in fishing. Population declines of more than 30% have been documented since 1956. They are restricted to 6 breeding islands, making them vulnerable to local stochastic events and guano collection and competition with Cape fur seals (Arctocephalus pusillus) have inhibited breeding at several islands. The global population is estimated at 150,000 pairs. Breeding colonies are protected currently and Morus capensis is protected in South Africa.
US Migratory Bird Act: no special status
US Federal List: no special status
CITES: no special status
IUCN Red List of Threatened Species: vulnerable
There are no negative impacts of Cape gannets on humans, although some fishermen persecute gannets because they are perceived to compete for fish prey. They do gather around commercial fishing and trawling boats to take advantage of fish aggregations.
Cape gannets are captured for food and fish bait, especially at breeding colonies. They continue to be collected in Angola, but are protected at breeding colonies in South Africa and Namibia. Breeding colonies have also been exploited for guano collection.
Positive Impacts: food ; body parts are source of valuable material; produces fertilizer
Cape gannets are important predators of pelagic and coastal fish off the coasts of Africa.
Cape gannets forage mainly over the continental shelf for fish, but sometimes enter pelagic waters to forage as well. They eat mainly shoaling fish, including pilchard (Sardinops sagax, up to 90% of the diet), anchovies (Engraulis encrasicolus), saury (Scomberesox saurus), mackerel (Scomber colias), and maasbankers (Trachurus). Fish are captured by plunge-diving, typically from about 20 meters high. Cape gannets also follow commercial fishing and trawling ships to take advantage of discarded fish, offal, and aggregations in nets. One study suggested that a pair of Cape gannets raising a hatchling to independence uses 246 kg of fish in a breeding season.
Animal Foods: fish
Primary Diet: carnivore (Piscivore )
Cape gannets are found in coastal areas of sub-Saharan Africa, from the Gulf of Guinea on the west coast to South Africa and to Mozambique, occasionally Kenya, on the east coast. They breed only off the coasts of South Africa and Namibia on approximately 6 breeding islands: Lambert's Bay, Malgas, and Algoa Bay in South Africa and Mercury, Ichaboe, and Possession Islands in Namibia.
Biogeographic Regions: ethiopian (Native ); indian ocean (Native ); atlantic ocean (Native )
Mating behaviors are not well-described in Cape gannets, but are similar to other gannet species, where ritualized displays are used to attract mates and maintain the pair bond. Gannets in general form pair bonds that last for life, with pairs reuniting at breeding colonies each year. Mates may meet several months before egg laying. During that time they use a rich suite of ritualized displays to re-establish the pair bond.
Mating System: monogamous
Cape gannets breed from September to April in dense colonies in open areas on flat or gently sloping, offshore islands, sometimes also on cliffs or human structures, such as guano platforms. They build elevated nests of dirt, vegetation, and guano, but eggs may also be laid on bare ground. Nests are typically placed very close to each other, within pecking distance of surrounding nests. Females lay 1, rarely 2, eggs in the central depression of the nest. Eggs are incubated for 44 days and young fledge at 97 days old. Cape gannets breed at 3 to 4 years old.
Breeding interval: Cape gannets breed once yearly.
Breeding season: Cape gannets breed from September to April.
Range eggs per season: 1 to 2.
Average eggs per season: 1.
Average time to hatching: 44 days.
Average fledging age: 97 days.
Range age at sexual or reproductive maturity (female): 3 to 4 years.
Range age at sexual or reproductive maturity (male): 3 to 4 years.
Key Reproductive Features: iteroparous ; seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; oviparous
Young Cape gannets hatch with some white down. After hatching, they are placed on the webbed feet of their parents and brooded continuously for a month, when they can regulate their own body temperature. Young are fed regurgitate by both parents until they become independent, some time after fledging. The length of the period of post-fledging independence is not reported in Cape gannets.
Parental Investment: altricial ; pre-fertilization (Provisioning, Protecting: Female); pre-hatching/birth (Provisioning: Female, Protecting: Male, Female); pre-weaning/fledging (Provisioning: Male, Female, Protecting: Male, Female); pre-independence (Provisioning: Male, Female)
The Cape gannet (Morus capensis) is a large seabird of the gannet family, Sulidae.
They are easily identified by their large size, black and white plumage and distinctive yellow crown and hindneck. The pale blue bill is pointed with fine serrations near the tip; perhaps because of the depth and speed of the gannet's dive when fishing (depending on altitude, gannets hit the water at speeds of between 40 and 120 km/h (25 and 75 mph)[2]), its beak has no external nostrils into which the water might be forced.
German naturalist Hinrich Lichtenstein described the Cape gannet in 1823.
The Sulidae, the gannets and boobies, appeared about 30 million years ago. Early Sulidae fossils most resembled the boobies, although they were more aquatic, with the gannets splitting off later, about 16 million years ago. The gannets evolved in the northern hemisphere, later colonising the southern oceans. The most ancient extant species may be the Abbott's booby, possibly the sole survivor of an otherwise extinct separate lineage.[3] A 2011 genetic study of nuclear and mitochondrial DNA suggests that the ancestor of the gannets arose around 2.5 million years ago before splitting into northern and southern lineages. The latter then splitting into the Cape and Australasian gannets around 0.5 million years ago.[4] The three gannets are generally considered to be separate species forming a superspecies, though they have also formerly been classified as subspecies of the northern gannet (Sula bassanus).[5]
When seen in flight the snow-white body with the black tail, primaries and secondaries, and dark bill makes them easy to identify. At closer range the distinctive golden crown and nape, which gradually becomes white on the neck, is noticeable. Juveniles and immatures are dark brown with a pale bill, and can resemble the dark-coloured boobies at first glance.
Adults are about 84–94 cm (33–37 in) long and have a 171–185 cm (67–73 in) wingspan and weigh ca. 2.6 kg (5.7 lb).
The Cape gannet is identified from the Australasian gannet by the all-black tail, a longer black stripe down the throat (from underneath the beak) and more extensive black on the face, but all of these features are difficult to identify unless at close proximity. The Cape gannet is also very similar to the northern gannet, but the latter differs from its entirely white tail and its wings, which are only black at the tip. It also is similar to the blue-footed booby (Sula nebouxii).
Gannet pairs may remain together over several seasons. They perform elaborate greeting rituals at the nest, stretching their bills and necks skywards and gently tapping bills together.
Cape gannets begin breeding in August or September. Typically the clutch is a single bluish egg, which soon becomes soiled. Both parents are actively involved in the incubation process which lasts for 42 to 46 days until hatching. Gannets use their foot webs to incubate the egg. The foot webs, which are richly irrigated with blood vessels are wrapped around the egg.
The hatchling is black, naked and blind, it weighs only about 70 g (2.5 oz), but within three weeks its body mass is one third of that of an adult. At eight weeks the chick outweighs the adult, and this remains so until it becomes a fledgling at 95–105 days of age.
Cape gannets are powerful fliers, using mainly a flap-gliding technique, which is more energy consuming than the dynamic-soaring favoured by albatrosses. As all Sulids, they are fish-eating birds that plunge-dive from considerable height.
The breeding range of the Cape gannet is restricted to southern Africa in three islands off Namibia and three islands off South Africa. They normally nest in large and dense colonies on flat islands or on flat ledges of the steeply sloping Mercury Island off Namibia. The world population was estimated in 1996 to number about 340,000 birds, with 12% in Namibia and 88% in South Africa. The largest colony of this bird, with over 140,000 birds, is found on Malgas Island, South Africa. Several birds have occasionally been found breeding on offshore Australian islands, together with Australasian gannets, although the Cape species is never represented by more than a few pairs.[6]
The non-breeding range of the Cape gannet extends from the coastal waters off the Gulf of Guinea on the west coast of Africa, to Mozambique on the east coast. They seldom occur farther offshore than 100 km, though records of birds more than 200 km offshore exist for both the Atlantic and Indian Oceans.
Numbers of Cape gannets at the Namibian islands have declined considerably between 1956 and 2000 from 114,600 to 18,200 breeding pairs respectively, an 84% decrease in less than fifty years. This contrasts with the trends at the South African islands where numbers have increased about 4.3 times during the same period, from 34,400 to 148,000 breeding pairs.
Vagrancy to the northern Atlantic Ocean has been reported for the Cape gannet on several occasions, but until 2016 was believed to be unproven.[7] Crochet and Haas[8] examined the status of the species in the western Palearctic realm. They list a number of historical claims which had not been accepted by national bird records committees, and analyse the single record which had hitherto been widely accepted, a juvenile recovered offshore from Western Sahara in 1966. In relation to the latter bird, they conclude that a mistake had occurred; the original data placed the bird inland, in Chad, with the offshore Western Saharan location being the result of an attempt to correct this to a more plausible location. A retransposition of the original latitude and longitude gives a location off the Namibian coast, well within the regular wintering range of the species. As a result, they recommended that the Cape gannet be deleted from the Western Palearctic list. However, on 14 April 2016, a Cape gannet was photographed at sea off Flores in the Azores.[9][10] The species has been discussed as a possible vagrant to Britain and Ireland, and there have been a number of claimed sightings, including one in December 2020 off the Isle of Wight, but there is yet to be an accepted record in the Western Palearctic apart from the Azores record.[11][12] One Spanish record from 1985 has been accepted as either a Cape gannet or an Australasian gannet.[11]
Individuals have also been photographed in other locations far from the species' usual range, including off Peru in 1999 and Oman in 2004.[11]
The Cape gannet (Morus capensis) is a large seabird of the gannet family, Sulidae.
They are easily identified by their large size, black and white plumage and distinctive yellow crown and hindneck. The pale blue bill is pointed with fine serrations near the tip; perhaps because of the depth and speed of the gannet's dive when fishing (depending on altitude, gannets hit the water at speeds of between 40 and 120 km/h (25 and 75 mph)), its beak has no external nostrils into which the water might be forced.
