Department of Statistical Sciences
University of Cape Town
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Avian Demography Unit Department of Statistical Sciences University of Cape Town |
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| BIRD NUMBERS | Volume 8 Number 1, June 1999 |
4. The avifauna of Dassen Island
Phil Whittington1 & Anton Wolfaardt2 Dassen Island lies 9 km off the west coast of South Africa, c. 60 km north of Cape Town. The nearest mainland settlement is Yzerfontein. The 222-ha island is largely flat, reaching only 19ÿm above sea level (Rand 1963). Vegetation cover varies with the seasons but is dominated by two shrubs: Tetragonia fruticosa and Trachyandra divaricata. During the dry summers, the vegetation dies back. The Trachyandra stems dry and detach and are carried all over the island and out to sea by the south-easterly and south-westerly winds. Their common name of 'Tumbleweed' is very apt as they roll across the island, piling up behind anything which acts as a windbreak. After the winter rains, the island appears much more lush as the vegetation regenerates and nettles and Arum Lilies appear. A few areas of temporary standing water develop after the rains, but these soon disappear as summer approaches. The island supports 34 regularly occurring bird species, of which 21 breed. This list includes some seasonal migrants and, in addition, there are occasional visitors and vagrants, as well as other species only recorded offshore. The island is important for its breeding seabirds. Recent counts of the Jackass Penguin Spheniscus demersus population have recorded c. 9000-15 000 nests, although there were thought to be around a million birds at the beginning of this century (Westphal & Rowan 1971; Shannon & Crawford submitted). Four species of cormorants breed on the island: Whitebreasted Cormorants Phalacrocorax carbo and Cape Cormorants P. capensis breed in spring and early summer while a small population of Bank Cormorants P. neglectus breeds chiefly in winter. The Crowned Cormorant P. coronatus can breed virtually all year round, but breeding activity peaks in midsummer. They breed on several shipwrecks around the island as well as on some wooden pedestals that were erected especially for this purpose. One of only two breeding colonies of White Pelicans Pelecanus onocrotalus in South Africa occurs on Dassen Island. The birds begin to lay in August and the chicks remain on the island into February before they are ready to leave. As well as bringing in food from the mainland, the adults will take nestlings of cormorants and Kelp Gulls Larus dominicanus on the island. Dassen Island has one of the two largest breeding colonies of the southern African subspecies vetula of the Kelp Gull. They breed in spring and early summer and make life somewhat unpleasant for other island dwellers. If you can imagine several thousand screaming gulls, occasionally bombing you with foul- smelling excreta, then you begin to get an idea of what living around Kelp Gulls is like. If you are near one of their nests - and it becomes difficult not to be as the season progresses - the adults will dive low over your head and may even make contact; this is somewhat similar to being hit over the head with a cricket bat! The somewhat more gently disposed although equally noisy Hartlaub's Gull L. hartlaubii nests during March and April, usually in close proximity to the colony of Swift Terns Sterna bergii. In November 1996, up to four Leach's Storm Petrels Oceanodroma leucorhoa were heard calling at night around the buildings at the north end of the island. Later the same month, breeding was confirmed for this species at Dyer Island, where at least 20 birds were thought to be occupying nest sites (Whittington et al. 1998). The species had hitherto been thought to be a nonbreeding visitor from the northern hemisphere. Further investigation at Dassen Island in October 1997 revealed that at least one bird was calling regularly from within a stone water tank. A bird was present in the same wall cavity in 1998, and while it is almost certainly breeding there, this has yet to be proved. Other seabirds are present offshore, Cape Gannets Morus capensis being present throughout the year, while Whitechinned Petrels Procellaria aequinoctialis and Sooty Shearwaters Puffinus griseus are more numerous during the winter months, along with an occasional Subantarctic Skua Catharacta antarctica, or perhaps a giant petrel Macronectes sp. During the summer, Arctic Skuas Stercorarius parasiticus can be seen offshore chasing terns, while in January and February, the elegant Sabine's Gull Larus sabini may grace the waters. In 1996, large numbers of these gulls were present around the island with as many as 1000 collecting in House Bay alone, on the island's northern shore. The peak count of 4400 was recorded by AW on 14 February, when the gull flock extended from the north-eastern point of the island, the 'Blaasbalk', along the entire eastern shore and round the southern coast. Hockey et al. (1989) stated that up to 4000 can occur in Table Bay, and Brian Cochrane recorded c. 2500 off Mouille Point in December 1977 (Cochrane 1978). Nine chicks of the Helmeted Guineafowl Numida meleagris were introduced to the island in December 1986 (B.M. Dyer pers. comm.). Their numbers have since increased to over 200. They gather to roost at night in the Manitoka trees around House Bay and the lighthouse. Up to four leucistic birds have been present since early 1996. These birds are a pale, pearly grey instead of the more normal dark grey colour (see Promerops 1995 No. 221 p.14). The present management policy is to eradicate this introduced species from the island, and numbers had been reduced to about 40 by September 1999. Other breeding birds on the island include Sacred Ibis Threskiornis aethiopicus, which nests in colonies amongst coastal boulders or on the ground amongst vegetation. Egyptian Geese Alopochen aegyptiacus breed in winter and spring, riding the surf together with their rafts of tiny goslings. Breeding waders include the African Black Oystercatcher Haematopus moquini, Whitefronted Plover Charadrius marginatus, Kittlitz's Plover C. pecuarius, Crowned Plover Vanellus coronatus and Blacksmith Plover V. armatus. Seasonal migrants occur in the shape of Palearctic waders and terns. The former consists of a few Grey Plovers Pluvialis squatarola, up to 120 Curlew Sandpipers Calidris ferruginea, c. 200 Sanderling C. alba and up to c. 400 Turnstones Arenaria interpres. These species are present during the summer months, although a small number of Turnstones can be found all year round. Visiting terns are mainly from the northern hemisphere. A few Sandwich Terns Sterna sandvicensis are usually present but Common Terns S. hirundo can reach high numbers with up to 4250 being present in February 1996. On 25 March 1997, there were approximately 7000 Common Terns feeding between the island and the mainland, many using the island as a temporary roosting site. In winter they are replaced by Antarctic Terns S. vittata, which commonly roost on the beach at House Bay, but these occur in much lower numbers. Other regular but nonbreeding visitors include Blackheaded Heron Ardea melanocephala and Grey Heron A. cinerea, Little Egret Egretta garzetta, Blackcrowned Night Heron Nycticorax nycticorax and Ringed Plover Charadrius hiaticula. Some species are irregular visitors from the mainland. Birds of prey are usually transient, the Blackshouldered Kite Elanus caeruleus being the most frequent. Yellowbilled Kite Milvus migrans parasitus and Peregrine Falco peregrinus have also been recorded. The most unusual record, however, must be of an immature Palmnut Vulture Gypohierax angolensis, which spent 3 or 4 days on the island in October 1997. In southern Africa, this species is normally found only on the KwaZulu-Natal coast, near Mtunzini, but occasional birds, particularly juveniles, are prone to wander. European Swallows Hirundo rustica and Little Swifts Apus affinis may put in an appearance in summer, while South African Shelduck Tadorna cana and Threebanded Plover Charadrius tricollaris may also occur from time to time. Most vagrants to the island are species which are fairly common on the mainland. These have recently included Spurwinged Goose Plectropterus gambensis, Redknobbed Coot Fulica cristata, Knot Calidris canutus, Greenshank Tringa nebularia, Bartailed Godwit Limosa lapponica, Curlew Numenius arquata, Greyheaded Gull Larus cirrocephalus, Greybacked Finchlark Eremopterix verticalis, Black Sawwing Swallow Psalidoprocne holomelas, Olive Thrush Turdus olivaceus, Paradise Flycatcher Terpsiphone viridis and Fiscal Shrike Lanius collaris. A national rarity turned up in the shape of an immature American Purple Gallinule Porphyrula martinica, which spent a week on the island in May 1996 before it died. A Rock Bunting Emberiza tahapisi, which was seen and photographed by AW, was present on the island on 8 and 9 May 1999. Only four species of landbirds can be regularly found on the island, all of them breeding. Rock Pigeons Columba guinea and House Sparrows Passer domesticus both utilize buildings for nesting, though the former also nest among rocks and on the various wrecks around the shore. House Sparrows also build their untidy nests in the Manitoka trees around House Bay and the lighthouse. European Starlings Sturnus vulgaris nest both in buildings and under Tetragonia shrubs. This is one of possibly very few places in South Africa where European Starlings use 'natural' nest sites. Cape Wagtails Motacilla capensis nest amongst the rocks and have even been found to nest in a whale skull. They gather to roost at dusk in the Manitoka trees at House Bay. Since 1997, a pair of Pied Crows Corvus albus have visited the island during the winter months. Their appearance seems to coincide with the abundance of young European Rabbits, on which they are regularly seen preying, at that time of year. In 1998, they built a nest on the observation platform at the top of the lighthouse, at the island's southern end. The nest was, however, later removed by lighthouse personnel. Cape Fur Seals rest on the island from time to time and Humpbacked Whales are regularly seen offshore, sometimes coming very close to the island. Recent vagrants have included a sick male Subantarctic Fur Seal, present on the island on 1 August 1995, and a young male Southern Elephant Seal, present for two days in September 1996. A Whale Shark, measuring 3.4 metres in length, was found washed up on the beach on 15 June 1995. We would like to thank Deon Nel and Bruce Dyer for their records of birds seen at Dassen Island. Marine and Coastal Management kindly provided transport to and from the island, while Cape Nature Conservation are thanked for providing accommodation and logistical support. Prof. Les Underhill and Bruce Dyer made valuable comments on earlier drafts of the manuscript.
Cochrane, B. 1978.
Sightings. Promerops 133: 4
Aerial photo of Dassen Island, looking westward. 
Dassen's lighthouse. |
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Document posted: 26 January 2000