Morus capensis (Cape Gannet)

Witmalgas [Afrikaans]; Umkholonjane [Xhosa]; Kaapse jan-van-gent, [Dutch]; Fou du Cap [French]; Kaptölpel [German]; Alcatraz do Cabo [Portuguese]

Life > Eukaryotes > Opisthokonta > Metazoa (animals) > Bilateria > Deuterostomia > Chordata > Craniata > Vertebrata (vertebrates)  > Gnathostomata (jawed vertebrates) > Teleostomi (teleost fish) > Osteichthyes (bony fish) > Class: Sarcopterygii (lobe-finned fish) > Stegocephalia (terrestrial vertebrates) > Tetrapoda (four-legged vertebrates) > Reptiliomorpha > Amniota > Reptilia (reptiles) > Romeriida > Diapsida > Archosauromorpha > Archosauria > Dinosauria (dinosaurs) > Saurischia > Theropoda (bipedal predatory dinosaurs) > Coelurosauria > Maniraptora > Aves (birds) > Order: Ciconiiformes > Family: Sulidae

Cape gannet. [photo Jeff Poklen ©]	Cape gannet juvenile off the coast of Cape Town, South Africa.[photo Trevor Hardaker ©]

Distribution and habitat

It breeds on islands along the coast of southern Africa, on Ichaboe and Possession Island (Namibia); Bird Island in Lambert's Bay and Malgas Island (Western Cape); Bird Island in Algoa Bay (Eastern Cape). In the non-breeding season, it disperses across the entire coast of southern Africa and northern Mozambique, also heading up the west Coast all the way to Ghana and Nigeria. It generally sticks fairly close to the coastline, while favouring islands with flat or sloping ground for nesting.

Predators and parasites

• Predators
  • Arctocephalus pusillus (Cape fur seal)
  • Orcinus orca (Killer whale)
• Parasites
  • Pasteurella multocida (Avian cholera)
  • Aspergillosus fumigatus (Saprophytic mould)
  • mites

Movements and migrations

Adults usually remain within 540 km of the island of their colony in winter, although they may travel up to 3300 km north. Immature birds move more freely however, following sardine shoals up to KwaZulu-Natal or alternatively heading north to West Africa, travelling up to about 6800 km: they even reach western Australia on occasion.

Food 

It mainly eats fish, doing most of its foraging in large flocks of up to about 1000 birds, searching for shoals of fish from the air. Once it locates prey it dives straight into the water, remaining submerged for 3-7 seconds, in which time it can descend up to 8 metres underwater using just momentum. It often associates with other seabirds, such as Cape cormorants, Kelp gulls and Sooty shearwaters, as well as dolphins, Cape fur seals (Arctocephalus pusillus) and gamefish. The following food items have been recorded in its diet:

Cape gannet. with caught fish, pelagic trip off of Cape Town, South Africa [photo Trevor Hardaker ©]

• Fish
  • Engraulis encrasicolus (Anchovy)
  • Sufflogobius bibarbatus (Pelagic goby)
  • Scomberesox saurus (Saury)
  • Merluccius (hakes)
  • Thyrsites atun (Snoek)
  • Trachurus trachurus (Horse mackerel)
  • Scomber japonicus (Chub mackerel)
  • Sardinops sagax (Sardine)
  • Etrumeus whiteheadi (Round herring)
• Cephalopods
  • Loligo vulgaris (Chokka squid)

Breeding

• Monogamous colonial nester, living in large and tightly packed colonies of up to about 68000 individuals. The sky-pointing display (seen in fig.1) is used as a greeting; if an individual tries to manoeuvre through the colony without holding its bill in the air, it is repeatedly pecked by other birds.
• The nest (see fig. 2) is a mound with a depression in the centre, made of guano and other material, which is applied to the structure by vibrating its mandible along the rim.

fig. 1 - Cape gannet colony, with two birds greeting each other with the sky-pointing display. [photo Jeff Poklen ©]
fig. 2 - Cape gannet, regurgitating food to chick. [photo H. Robertson ©]	fig. 3 - Cape gannets on their nests. [photo Peter Steyn ©]

• Egg-laying season can be year-round, peaking from September-April.
• It lays a single egg, rarely two, which is incubated by both sexes for about 43-44 days.
• The chick is fed by both sexes by regurgitation (see fig. 2) and brooded for the first 2-3 days of its life. It leaves the nest at 90-105 days old, remaining on the colony edge for another six days or so before dispersing completely.

Threats

Vulnerable, as its population has decreased by at least 20% in three generations, and is especially in trouble in Namibia. This is largely due to the collapse of Sardine (Sardinops sagax) and other fish stock, as well as occasional oil spills which cause hundreds of deaths.

References

• Hockey PAR, Dean WRJ and Ryan PG 2005. Roberts - Birds of southern Africa, VIIth ed. The Trustees of the John Voelcker Bird Book Fund, Cape Town. 

• Harrison, J.A., Allan, D.G., Underhill, L.G., Herremans, M., Tree. A.J., Parker, V. & Brown, C.J. (eds). 1997. The atlas of southern African birds. Vol. 2: Passerines. BirdLife South Africa, Johannesburg.