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Morus capensisCape gannet

By Tanya Dewey

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. (BirdLife International, 2009; del Hoyo, et al., 1992)

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. (BirdLife International, 2009; del Hoyo, et al., 1992)

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. (BirdLife International, 2009; del Hoyo, et al., 1992)

  • Sexual Dimorphism
  • sexes alike
  • Average mass
    2600 g
    91.63 oz
  • Range length
    85 to 94 cm
    33.46 to 37.01 in

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. (del Hoyo, et al., 1992)

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. (BirdLife International, 2009; del Hoyo, et al., 1992)

  • 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

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. (del Hoyo, et al., 1992)

  • 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

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. (del Hoyo, et al., 1992)

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. (BirdLife International, 2009)

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. (BirdLife International, 2009; del Hoyo, et al., 1992)

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. (BirdLife International, 2009; del Hoyo, et al., 1992)

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 sagax, 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. (Adams, et al., 1991; del Hoyo, et al., 1992)

  • 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. (BirdLife International, 2009)

Ecosystem Roles

Cape gannets are important predators of pelagic and coastal fish off the coasts of Africa. (del Hoyo, et al., 1992)

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. (BirdLife International, 2009; del Hoyo, et al., 1992)

  • Positive Impacts
  • 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. (del Hoyo, et al., 1992)

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. (BirdLife International, 2009; del Hoyo, et al., 1992)

Other Comments

Cape gannets were previously known as Sula capensis and Dysporus capensis. (BirdLife International, 2009; del Hoyo, et al., 1992)

Contributors

Tanya Dewey (author), Animal Diversity Web.

Glossary

Atlantic Ocean

the body of water between Africa, Europe, the southern ocean (above 60 degrees south latitude), and the western hemisphere. It is the second largest ocean in the world after the Pacific Ocean.

World Map

Ethiopian

living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.

World Map

acoustic

uses sound to communicate

altricial

young are born in a relatively underdeveloped state; they are unable to feed or care for themselves or locomote independently for a period of time after birth/hatching. In birds, naked and helpless after hatching.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

carnivore

an animal that mainly eats meat

chemical

uses smells or other chemicals to communicate

coastal

the nearshore aquatic habitats near a coast, or shoreline.

colonial

used loosely to describe any group of organisms living together or in close proximity to each other - for example nesting shorebirds that live in large colonies. More specifically refers to a group of organisms in which members act as specialized subunits (a continuous, modular society) - as in clonal organisms.

diurnal
  1. active during the day, 2. lasting for one day.
endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

food

A substance that provides both nutrients and energy to a living thing.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

monogamous

Having one mate at a time.

motile

having the capacity to move from one place to another.

natatorial

specialized for swimming

native range

the area in which the animal is naturally found, the region in which it is endemic.

nomadic

generally wanders from place to place, usually within a well-defined range.

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

pelagic

An aquatic biome consisting of the open ocean, far from land, does not include sea bottom (benthic zone).

piscivore

an animal that mainly eats fish

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

seasonal breeding

breeding is confined to a particular season

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

social

associates with others of its species; forms social groups.

tactile

uses touch to communicate

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

visual

uses sight to communicate

References

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

Energy expenditure and food consumption by breeding Cape gannets Morus capensis
. Marine ecology progress series, 70: 1-9.

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.

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