Hartlaub's Gull Larus hartlaubii
Text from: Undehill, L.G., Tree, A.J., Oschadleus, H.D. & Parker, V. 1999.
Review of ring recoveries of waterbirds in southern Africa.
Cape Town: Avian Demography Unit, university of Cape Town. Pp. 93-95.
Once considered a subspecies of the Silver Gull Larus novaehollandiae of Australasia, Hartlaub's Gull currently has species status and is a southern African endemic. It occurs along the coastline from Quoin Point in the Western Cape to Cape Cross in Namibia; there are records both to the north and east. It is regarded as sedentary, undertaking local movements. The total population is estimated to be c. 30 000 birds, making it about the 10th rarest of the c. 50 gull species in the world (ASAB1 466–467; Rose & Scott 1997).
Photo L.G. Underhill, 2000
It occurs along the coastline, with concentrations at each centre of human habitation within the range, especially those with a port or a fishing industry. It has expanded its range inland into cities, towns, suburbs and farmland. It is attracted to artificial wetlands, flooded areas where earthworms and insects come to the surface, rubbish dumps and other places where food is discarded. In common with many other gull species, it has increased in abundance, and has exploited new breeding sites, such as quarries, airfields and buildings. It breeds at the sewage works at Paarl, c. 50 km from the coast (e.g. Brown 1990).
There are 394 recoveries from 10 299 birds ringed (Table 71, Fig. 58). Most ringing took place during two periods. Between 1955 and 1971, walk-in traps were used at Rondevlei Bird Sanctuary and 1386 flying birds were caught. Between 1968 and 1981, 7873 fledglings were ringed at the breeding colony at Robben Island (Underhill & Underhill 1986). In some years, fledglings were also colour-ringed.
80% of the recoveries were within 30 km of the ringing site; most of these were movements within the Greater Cape Town area, where many of the recoveries were made in residential and industrial areas some distance from the coast (e.g. 3 & 8). Beyond the suburbs of Cape Town, only two recoveries were not on the coast: adults ringed at Rondevlei moved to Malmesbury (111 km due north after 16 months) and to Paarl (57 km northeast after 7 y 7 m) (11). Coastal recoveries ranged between 228 km north of Robben Island along the West Coast to the Olifants River mouth (1), and 137 km eastward to Dyer Island and Pearly Beach in the direction of Cape Agulhas (10). However, colour-ringed gulls from Robben Island have been observed as far north as Swakopmund in Namibia (LGU unpubl. data). There is an equivocal record of a blue colour-ringed gull at Lake St Lucia in northern KwaZulu-Natal less than a year after blue colour rings were used on Robben Island; it was identified as a Grey-headed Gull L. cirrocephalus, but no colour-ringing of this species had ever taken place at the time (Anon. 1974; Elliott 1974c).
Underhill & Underhill (1986) considered that about 40% of fledglings survived the i mmediate post-fledging period and that this proportion varied considerably between years. They estimated the survival rate for the remainder of the first year to be 0.76, and that the annual survival rates were 0.79 and 0.87 for second-year and adult birds, respectively. Applying these survival rates to a cohort of 7873 fledglings results in 177 birds reaching ages exceeding 20 years, of which only a small proportion would be recovered and reported to SAFRING. There are, in fact, four birds, all ringed as chicks on Robben Island, with recovery ages exceeding 20 years (e.g. 2, 3 & 4). The oldest of these was 23 y 8 m (2).
There is a need to update the paper by Underhill & Underhill (1986), using more sophisticated methods to estimate survival rates. The colour-ringing study during the 1970s failed to produce useful results because the colour-rings failed after a few years (LGU unpubl. data); a new colour-ringing study using individually engraved rings was launched during 1999 (LGU, R.J.M. Crawford).
|No.||Ring||Ring date||Age||Ringing coords.||Ringing place||Finding date||Finding coords.||Finding place||Elapsed||Dist||Code|
|1||507844||11.05.74||N||3348 1823||Robben I, WC||12.05.75||3145 1814||Olifants R, WC||1y 0m||228||648|
|2||517315||20.05.72||N||3305 1801||Schaapen I, WC||12.01.96||3305 1800||Langebaan, WC||23y 8m||2||644|
|3||517441||12.06.71||N||3348 1823||Robben I||13.04.93||3352 1833||Bellville, WC||21y 10m||17||276|
|4||523422||11.05.74||N||3348 1823||Robben I||14.12.97||3205 1817||Lambert's Bay, WC||23y 7m||191||643|
|5||532036||18.05.74||N||3348 1823||Robben I||09.02.78||3149 1814||Doring Bay, WC||3y 9m||221||648|
|6||55309242||01.10.59||Ad||3404 1830||Rondevlei NR, WC||26.08.60||3257 1753||Vredenburg, WC||10m 26d||137||199|
|7||560715||24.05.75||N||3348 1823||Robben I||07.08.76||3435 1921||Gansbaai, WC||1 y 2m||133||599|
|8||560803||24.05.75||N||3348 1823||Robben I||21.03.95||3356 1829||Cape Town, WC||19y 10m||18||299|
|9||560994||31.05.75||N||3348 1823||Robben I||16.12.75||3425 1912||Hermanus, WC||6m 16d||116||199|
|10||571433||11.05.81||N||3348 1823||Robben I||28.05.92||3441 1925||Dyer I, WC||11y 1m||143||299|
|11||64505326||24.02.68||Ad||3404 1830||Rondevlei NR||15.09.75||3344 1858||Paarl, WC||7y 7m||57||754|
Fig. 58. Ringing and recovery sites for Hartlaub's gull.