Morus capensis

Geographic Range

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 ethiopian
  • native native
• indian ocean
  • native native
• atlantic ocean atlantic ocean
  • native native

Habitat

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 temperate
• tropical tropical
• saltwater or marine saltwater or marine

• Aquatic Biomes
• pelagic pelagic
• coastal coastal

Physical Description

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.

• Other Physical Features
• endothermic endothermic
• homoiothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• sexes alike

Reproduction

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 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.

• Key Reproductive Features
• iteroparous iteroparous
• seasonal breeding seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• oviparous 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 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

Lifespan/Longevity

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.

Behavior

Cape gannets are social and sedentary. Juveniles disperse long distances (~4000 km) northwards after fledging, remaining in northern areas for a year after hatching. Adults tend to stay in the same general area, typically within 500 km of their breeding island. Some adults, however, wander over 3000 km outside of the breeding season. Cape gannets are active during the day.

• Key Behaviors
• flies
• natatorial natatorial
• diurnal diurnal
• motile motile
• nomadic nomadic
• sedentary sedentary
• social social
• colonial colonial

Home Range

Cape gannets occasionally wander widely to forage, but generally remain within 500 km of breeding islands. Nests at breeding colonies are very close together, within pecking distance of neighbors.

Communication and Perception

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 visual
• acoustic acoustic

• Perception Channels
• visual visual
• tactile tactile
• acoustic acoustic
• chemical chemical

Food Habits

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 ocellata , up to 90% of the diet), anchovies ( Engraulis capensis ), saury ( Scomberesox saurus ), mackerel ( Scomber japonicus ), 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.

• Primary Diet
• carnivore carnivore
  • piscivore piscivore

• Animal Foods
• fish

Predation

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.

Ecosystem Roles

Cape gannets are important predators of pelagic and coastal fish off the coasts of Africa.

Economic Importance for Humans: Positive

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 food
• body parts are source of valuable material
• produces fertilizer

Economic Importance for Humans: Negative

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.

Conservation Status

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 capensis ). 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.

Other Comments

Cape gannets were previously known as Sula capensis and Dysporus capensis .

Additional Links

Encyclopedia of Life

Contributors

Tanya Dewey (author), Animal Diversity Web.

References

Adams, N., R. Abrams, W. Siegfried, K. Nagy, I. Kaplan. 1991.

Energy expenditure and food consumption by breeding Cape gannets Morus capensis

BirdLife International, 2009. "Species factsheet: Morus capensis" (On-line). BirdLife International. Accessed July 21, 2009 at http://www.birdlife.org/datazone/species/index.html?action=SpcHTMDetails.asp&sid=3653&m=0 .

del Hoyo, J., A. Elliott, J. Sargatal. 1992. Handbook of the Birds of the World, Volume I . Barcelona: Lynx Edicions.