Demography and Populations

During the breeding season, large flocks of non-breeding birds can still be observed, mainly comprising pairs (109 Siegfried, W. R. (1976). Sex ratio in the Cape Shelduck. Ostrich 47:113–116. Close , 1 Geldenhuys, J. N. (1980). Breeding ecology of the South African shelduck in the southern Orange Free State. South African Journal of Wildlife Research 10:94–111. Close ), although it has also been alleged that such flocks comprise chiefly females (22 Taylor, J. S. (1944). Notes on the South African Shelduck. Ostrich 15:188–193. Close ). In Free State Province the pre-breeding period (early winter) is characterized by the population being split into sedentary territorial pairs, nomadic non-territorial pairs, and flocks of non-breeders (1 Geldenhuys, J. N. (1980). Breeding ecology of the South African shelduck in the southern Orange Free State. South African Journal of Wildlife Research 10:94–111. Close ). Territorial adults comprised 69.0% of birds at one site and 74.4% at another during this time, relative to the total adult population. Sightings of solitary individuals usually (90%) involved females.

Age at First Breeding

No information (29 Allan, D. G. (2005). South African Shelduck Tadorna cana. In Roberts Birds of Southern Africa. Seventh Edition. (P. A. R. Hockey, W. R. J. Dean, and P. G. Ryan, Editors), The Trustees of the John Voelcker Bird Book Fund, Cape Town. pp. 93–95. Close ).

Annual and Lifetime Reproductive Success

At Graaff-Reinet, Eastern Cape Province, up to 13 chicks were seen in a brood, with a mean brood size of 7.6 (n = 15) (22 Taylor, J. S. (1944). Notes on the South African Shelduck. Ostrich 15:188–193. Close ). In Free State Province, mean brood size decreased from a mean 7.4 for one-week-old broods to 5.8 for broods at fledging (70 days) (n = 139 broods monitored), i.e., a loss of 1.6 ducklings per brood across the fledging period, excluding seven broods that were entirely lost (5%), six when 7–14 days old, and one when 14–21 days old (1 Geldenhuys, J. N. (1980). Breeding ecology of the South African shelduck in the southern Orange Free State. South African Journal of Wildlife Research 10:94–111. Close ). The largest broods had the lowest rate of chick loss. Brood sizes ranged between one and 21 chicks (129 Geldenhuys, J. N. (1976). Breeding status of waterfowl at large lakes in the Orange Free State. Ostrich 47:137–139. Close ). There is one record of a pair with 28 downy young all of the same size but it is suggested that some pairs may ‘kidnap’ the young of others (130 Brooke, R. K. (1989). A large South African Shelduck brood or a creche? Promerops 191:14. Close ).

Breeding success was high in Free State Province, and of 139 broods which safely arrived at water from their breeding burrows, only seven were entirely lost before fledging (1 Geldenhuys, J. N. (1980). Breeding ecology of the South African shelduck in the southern Orange Free State. South African Journal of Wildlife Research 10:94–111. Close ). Of the remainder, attrition pre-fledging accounted for only an average of 1.6 chicks per brood, i.e., a reduction in mean brood size from 7.4 to 5.8. Most mortality occurred in the first four weeks. Survival to fledging was slightly, but significantly, higher in a Karoo study (76.4%) compared to a grassland study area (71.5%), due to higher mortality during the first two weeks in the latter region. Examining all territorial pairs, the mean number of young successfully reared to fledging per annum was 5.0, with 86% of all pairs breeding successfully. High breeding success was attributed to the strongly developed pair bond, territoriality, and social hierarchy among age classes. Late broods were no less successful than early broods, but pairs that established territories earliest (but did not necessarily lay immediately) had larger brood sizes, and suffered lower rates of chick loss, than pairs establishing territories later in the highly circumscribed breeding season.

Birds captured for ringing at Barberspan comprised only 4% juveniles of the year in the post-breeding period (n = 390), but this figure is unlikely to be representative of breeding success as this site is apparently distant from the main breeding areas, and most juveniles avoid wing-molting grounds, instead apparently remaining close to their natal sites during their first year (2 Shewell, A. L. (1959). The waterfowl of Barberspan. Ostrich Supplement 3:160–179. Close ). At Vogelvlei 23% of birds captured during the post-breeding period were juveniles (n = 209) and little wing-molting occurs at this site (102 Siegfried, W. R. (1967). Trapping and ringing of Egyptian Geese and African Shelduck at Vogelvlei, Cape. Ostrich 38:173‒178. Close ). The high density of breeding pairs present in the wetter eastern parts of the range suggest that this region is a particularly important breeding ground, relative to the drier west (1 Geldenhuys, J. N. (1980). Breeding ecology of the South African shelduck in the southern Orange Free State. South African Journal of Wildlife Research 10:94–111. Close ).

Number of Broods Normally Reared per Season

Apparently single-brooded, but may replace failed clutches or broods lost when chicks are small (22 Taylor, J. S. (1944). Notes on the South African Shelduck. Ostrich 15:188–193. Close , 103 Geldenhuys, J. N. (1980). Breeding seasons of Egyptian Geese and South African Shelducks in Central South Africa. In Proceedings IV Pan-African Ornithological Congress (D. N. Johnson, Editor), Southern African Ornithological Society. pp. 267–275. Close ).

Adult populations remain relatively stable, despite wide annual variations in habitat availability. Viewed in the light of the species’ high breeding productivity, this suggests that full-grown birds suffer high mortality rates (1 Geldenhuys, J. N. (1980). Breeding ecology of the South African shelduck in the southern Orange Free State. South African Journal of Wildlife Research 10:94–111. Close ). From ringing information, the longevity record is 13 years 11 months, with five additional records exceeding ten years (99 Underhill, L. G., A. J. Tree, H. D. Oschadleus and V. Parker (1999). Review of Ring Recoveries of Waterbirds in Southern Africa. Avian Demography Unit, University of Cape Town, Cape Town, South Africa. Close ). One captive bird lived for more than 40 years (131 Duijzend, P. (1969). News and views. The Avicultural Magazine 75:276. Close ).

Susceptible to botulism (Clostridium botulinum, type C) and at two sites 18–19% mortality was registered during outbreaks (132 Blaker, D. (1967). An outbreak of botulinus poisoning among waterbirds. Ostrich 38:144–147. Close , 133 van Heerden, J. (1974). Botulism in the Orange Free State goldfields. Ostrich 45:182–184. Close ). Captive birds are prone to tuberculosis, aspergillosis, and amidostomiasis (gizzard worm) (134 Hillgarth, N., and J Kear (1979). Diseases of sheldgeese and shelducks in captivity. Wildfowl 30:142–6. Close ). One bird probably died from avian cholera (Pasteurella multoceda) at Jutten Island in Saldanha Bay, Western Cape Province, in October 1991 (135 Crawford, R. J. M., D. M. Allwright, and C. W. Heÿl (1992). High mortality of Cape Cormorants (Phalacrocorax capensis) off western South Africa in 1991 caused by Pasteurella multocida. Colonial Waterbirds 15:236–238. Close ). Avian influenza virus was isolated from the gut and organs of one of eight birds sampled at Oudtshoorn, Western Cape (136 Pfitzer, S., D. J. Verwoerd, G. H. Gerdes, A. E. Labuschagne, A. Erasmus, R. J. Manvell, and C. Grund (2000). Newcastle disease and avian influenza A virus in wild waterfowl in South Africa. Avian Diseases 44:655–660. Close ). Avian influenza virus was also identified in two additional individuals (H5 and H6 strains) (137 Cumming, G. S., A. Caron, C. Abolnik, G. Cattoli, L. W. Bruinzeel, C. E. Burger, K. Cecchettin, N. Chiweshe, B. Mochotlhoane, G. L. Mutumi, and M. Ndlovu (2011). The ecology of Influenza A viruses in wild birds in southern Africa. EcoHealth 8:4–13. Close ). It is considered a fairly high-risk species for the transmission of avian influenza (138 Cumming, G. S., P. A. R. Hockey, L. W. Bruinzeel, and M. A. Du Plessis (2008). Wild bird movements and avian influenza risk mapping in southern Africa. Ecology and Society 13(2):26. Close ).

Information needed.

Territory Size

Territory sizes comprise 75–170 m of shoreline and a distance inward to the center of wetlands of 50–80 m, i.e., about 0.5 ha of water (n = 24) (1 Geldenhuys, J. N. (1980). Breeding ecology of the South African shelduck in the southern Orange Free State. South African Journal of Wildlife Research 10:94–111. Close ). At larger wetlands several territories may abut one another.

Numbers

In some parts of its range, South African Shelduck can be the most abundant waterfowl species, e.g., in southern and western Free State Province (98 Geldenhuys, J. N. (1976). Relative abundance of waterfowl in the Orange Free State. Ostrich 47:27–54. Close ), and overall at large irrigation dams throughout the Free State it was the second-most common of 15 waterfowl species, after the Egyptian Goose (Alopochen aegyptiaca) (97 Geldenhuys, J. N. (1975). Waterfowl (Anatidae) on irrigation lakes in the Orange Free State. Ostrich 46:219–235. Close , 98 Geldenhuys, J. N. (1976). Relative abundance of waterfowl in the Orange Free State. Ostrich 47:27–54. Close ). The total population is estimated at about 50,000 (Wetlands International 2021).

At 23 known molt localities in South Africa, mainly large dams, 16 of which were in the Free State, 30,000 individuals were estimated to occur in the 1980s, or at least 70% of the South African population, the country in which the vast majority of the global population occurs (10 Geldenhuys, J. N. (1981). Moults and moult localities of the South African Shelduck. Ostrich 52(3):129–134. Close ). The six largest dams (Vaal, Bloemhof, Allemanskraal, Erfenis, Krugersdrift, and Kalkfontein) supported 78% of these birds. There are two additional major molting localities in Western Cape Province (Radyn Dam and De Hoop Vlei, the latter a natural wetland) (93 Hockey, P. A. R., L. G. Underhill, and M. Neatherway (1989). Atlas of the Birds of the Southwestern Cape. Cape Bird Club, Cape Town, South Africa. Close ). The largest flocks are present at these molting localities, with up to about 5,000 birds (25 Brown, L. H., E. K. Urban, and K. Newman (1982). The Birds of Africa. Volume 1. Academic Press, London, UK. Close ). Recent (April 2021) data from the biannual South African Coordinated Waterbird Counts Project (CWAC 2021) lists 20 sites in Free State, six in Western Cape and two in Northern Cape Province, as having recorded maximum numbers exceeding the Ramsar threshold of 500 individuals for this species (mean and maximum numbers in brackets): Free State: Allemanskraal Dam (76; 611), Bloemhof Dam (200; 2,854), Danielskuil Pan (265; 2,762), Deelpan (616; 6,447), Donkerpoort Farm Dam 1 (108; 611), Erfenis Dam (141; 1,344), Gariep Dam (270; 840), Kalkfontin Dam (504; 2,721), Knellpoort Dam (42; 644), Krugersdrift Dam (483; 3,545), Leeupan (239; 879), Vaalbank Dam (417; 3,263); Western Cape: De Hoop Vlei (84; 730), Koenskraal Pan (295; 675), Radyn Dam (334; 1,750), Verlorenvlei (132; 501), Voelvlei (461; 1,490), Wadrif Saltpan (98; 1,365); Northern Cape: Orange River Mouth (180; 678) and Spitskop Dam (534; 2,926). The total mean number of birds recorded at these 28 sites is 5,479 individuals, or 46% of the total mean number recorded at all 371 wetlands covered by this project. Biannual counts such as these, however, can overlook large temporary influxes , e.g., a temporary influx of 1,412 molting birds to De Hoop Vlei in 2015 (139 De Blocq van Scheltinga, A. (2017). Disturbance effects of boat-based tourism on waterbirds at the Ramsar-designated De Hoop Vlei, Western Cape, South Africa. M.S. thesis, University of Cape Town, Cape Town, South Africa. Close ). The population inhabiting the former Transvaal Province (northeast South Africa) was estimated at ca. 5,000 birds in the mid-1980s (126 Tarboton, W. R., M. I. Kemp, and A. C. Kemp (1987). Birds of the Transvaal. Transvaal Museum, Pretoria, South Africa. Close ). It is estimated that 15% of the global population occurs in Namibia (140 Brown, C. J., J. M. Mendelsohn, N. Thomson, and M. Boorman (2017). Checklist and analysis of the birds of Namibia as at 31 January 2016. Biodiversity Observations 8.20:1–153. Close ). Up to ca. 90 birds have been counted at regular sites in southeastern Botswana, and the total population in this region is estimated at about 200–300 birds (49 Tyler, S. J. (2020). Changing status of South African Shelduck (Cape Shelduck) Tadorna cana in Botswana. Babbler 66:4–10. Close ). The two most regular sites in Lesotho — Letšeng-la-Letsie and Tša-Kholo — support up to about 30 individuals each, and the total Lesotho population is thought not to exceed 100 birds (51 Ambrose, D. (2020). Lesotho Annotated Bibliography, Section 167A, Birds – Annotated Bird Checklist. House 9 Publications and Mohokane Trust, Roma, Lesotho. Close ).

Trends

The South African Shelduck has undoubtedly increased in abundance both within its ancestral range and by expansion into new regions (see under Historical Changes to the Distribution and under Conservation Status).