Repeated observations of a Cape Gannet Morus capensis on the coast of Patagonia, Argentina

REBSTOCK, G.A.; AGÜERO, M.L.; BOERSMA, P.D.; EBERT, L.; GÓMEZ LAICH, A.; LISNIZER, N.; SVAGELJ, W.; TRIVELLINI, M.

OSTRICH

NATL INQUIRY SERVICES CENTRE PTY LTD

Año: 2010 vol. 81 p. 167 - 169

0030-6525

The Cape Gannet (Morus capensis) is a seabird endemic to southern Africa, breeding at only six localities, three in Namibia and three in South Africa. The population is currently estimated to be 151 000 pairs. After the breeding season, some Cape Gannets disperse north, along the west coast of Africa, while many follow sardine (Sardinops sagax) schools eastward. The species is usually restricted to the continental shelf of Africa and rarely seen more than a few hundred kilometers offshore. In spite of this relatively restricted distribution, vagrants of this species have been reported in Australia, New Zealand, Amsterdam Island (Indian Ocean), Spain, Argentina, Brazil, and Perú. During the 2007/2008 and 2008/2009 austral breeding seasons an adult Cape Gannet was sighted by several people at Punta Tombo (44º03'S, 65º18'W) and Punta León (43º05'S, 64º30'W), Argentina. These points are about 125 km apart. In both locations, the bird was identified as an adult gannet by its shape, white body plumage, yellowish-buff head, and green lines on the legs and feet. The bird was seen perched and in flight, and had black tail feathers, black secondaries, black primaries, a long black gular stripe, and a pale iris, all diagnostic characteristics of a Cape Gannet. The Cape Gannet was always close to or in a colony of Imperial Cormorants (Phalacrocorax atriceps) or a mixed colony of Imperial Cormorants and Rock Shags (P. magellanicus). During the 2008 sighting at Punta León, we observed the Cape Gannet interacting with Imperial Cormorants. The gannet displaced two Imperial Cormorant females from their nests. The gannet, without aggressive behaviour, occupied the nests, and the cormorants attempted to defend their nests by pecking. The mismatch of size and mass ? approximately 2 650 g for the gannet and 1 950 g for the female cormorant ? resulted in the smaller cormorants surrendering their nests for approximately 30 minutes. One nest contained two cormorant chicks of about 5 d of age, which were ejected from the nest by the gannet. The other nest was under construction and contained no eggs or chicks. In the other sightings, the gannet remained between nests and did not attempt to occupy a nest. Cape Gannets have been reported on both coasts of South America, especially off Brazil and Argentina. The Cape Gannets in Brazilian waters were seen in the austral late summer, autumn, or winter (March?August), generally outside the Cape Gannet breeding season. Cape Gannets in Argentine waters were seen in austral spring and summer (October?January), corresponding to the Cape Gannet breeding season. Cape Gannets typically lay eggs between mid-September and mid-December, with chicks fledging from mid-January to April. Our sightings, in November and December, correspond to the incubation and early chick-rearing periods of Cape Gannets in Africa. Our sightings of only one Cape Gannet at a time within a few weeks of each other in 2007 and 2008 were probably of the same individual. We also think it likely that it was the same individual in both years at the cormorant colonies. Adult survival is 0.93 in Cape Gannets with breeding adults having an average longevity of 13?14 y. Argentine anchovy (Engraulis anchoita), which supports seabird, pinniped, and predatory fish populations on the Patagonian shelf, is related to the anchovy (Engraulis encrasicolus) in southern African waters that is one of the primary prey species of Cape Gannets there. A Cape Gannet could survive on anchovy in Argentine waters.