Mallards, why they should go
Mallard ducks (Anas platyrhynchos) are listed as an alien invader species in the South African National Scientific Programs Report No 113 (1985). This is a Red Data book, a series of books that list both endangered species, and species that are declared invaders. Likewise alien tree species, such as Port Jackson (Acacia saligna) and Rooikrans (Acacia cyclops) are listed in Report No 85 (1984). These alien tree species pose a threat to biodiversity in South Africa and are being actively removed. For comparison, mallard ducks can be considered the Port Jackson’s of the avian world.
The recently completed Atlas of Southern African Birds has this to say about the presence of Mallards in the Southern African Region “A localized feral resident in the southwestern Cape Province and on the Witwatersrand” (Cohen 1997).
Since the early 1980’s numbers have increased in some areas. (Cohen 1997) e.g. the numbers at Zandvlei increased from a few individuals in the early 1980’s to at least 250 in the early 1990’s (pers. Obs.) Its hybridization with other Anas species has been documented in many parts of the world and in southern Africa it hybridizes with the yellow-billed duck (Cohen 1997).
Hybrids have been observed in the South-western and Eastern Cape Province. On the Witwatersrand the increase in the numbers of Mallards is a cause for concern, as hybridization may pose a long-term conservation threat to the genetic integrity of the yellow-billed duck” (Cohen 1997).
The fact that Mallard duck is an aggressive breeder, hybridizing with numerous other waterfowl species worldwide, is well documented.
Please inform your nearest nature conservation office of unauthorized keeping of Mallards or of Mallards occurring in nature.
Threats posed by Mallards and other feral waterfowl in Cape Town
There is a growing concern in South Africa about dangers posed to indigenous biota by introduced ‘alien’ species. Such threats include competitive displacement, hydrological interference and hybridization. The latter problem is potentially severe when hybrid offspring are fertile.
Relative to plant species, there have been few introductions of alien bird species into South Africa. The three most successful alien birds, House Sparrow, European Starling and Indian Myna have almost certainly displaced some indigenous species locally, but have not hybridized with any indigenous species.
The Mallard (Anas platyrhynchos) is a popular ornamental waterfowl, being attractive, tame and easy to feed and breed.
Mallards are completely dominant to, and known to hybridize with many other waterfowl species. The question has been raised on several occasions, as to whether more vigorous control should take place to minimize potentially damaging effects on indigenous waterfowl, either through displacement or genetic ‘pollution’.
Mallards inhabit a wide range of aquatic habitats. They are tolerant of man and associated disturbance and are common, urban birds in many parts of their native range. Concomitant with their wide habitat tolerance, they also have a wide diet spectrum.
The Marina da Gama Situation
Surveys of waterfowl were made at the following water bodies: Rietvlei, Zekoevlei, Zandvlei, Rondevlei, Pricess and Little Princess Vleis, Dreyersdal Farm dam and Strandfontein Sewage works.
During a survey of the waterways at Marina da Gama, I counted 26 ‘white farmyard ducks’ (known locally as ‘Dutch Quackers’); 27 birds (18 males, 9 females) which appeared to be pure, or nearly pure Mallards; and 35 birds (24 males, 9 females, 2 of unknown sex) which were Mallard x Dutch Quacker hybrids (total = 88 birds). The only indigenous ducks recorded in the area were one pair of Cape Teals. Several of the feral ducks were paired (presumably prior to breeding). One pair was accompanied by 13 recently hatched ducklings.
On the basis of these observations it can be concluded with fair certainty that a) most, if not all, of the ‘Mallards’ in the Cape Town area are themselves hybrids, b) these hybrids are reproductively active and c) the ducks concentrate in areas where they are fed regularly (almost invariably in waterside residential areas).
The lack of indigenous ducks in the southern and eastern sections of Zandvlei may be due in part to the presence of Mallards, but also undoubtedly reflects a) the high level of disturbance and b) the destruction of natural riparian vegetation.
The ecological threats posed by Mallards, other than those of species displacement and hybridization, are difficult to quantify because of the opportunistic feeding and wide habitat tolerance of Mallards. At present, much of their food is provided by man, & is thus obtained from outside the vlei ecosystem. As a result, their droppings will constitute a net of nutrient input to the water body. The input of soluble inorganic salts, such as NO2, NO3 and PO4, will contribute towards eutrophication of the system. An immediate effect of such input will stimulate the algal production in the channels at Marina da Gama where the birds spend most of their time. These effects may lead to increased populations of herbivorous fishes and birds (specifically Red knobbed Coots).
The existing feral population in Cape Town could be controlled relatively easily and quickly because the birds aggregate at feeding stations. However, such control measures are likely to lead to considerable resistance from ‘duck-loving’ residents. In view of this, a partial control may be preferable, in which only those birds exhibiting full or partial Mallard plumage are removed. The object would be to leave only pure white ‘Dutch Quackers’. It is likely that some offspring of these white ducks will have Mallard plumage characteristics. Such offspring should be removed (annually, before they have opportunity to breed). Under such a control programme, the frequency of such ‘throwbacks’ will gradually diminish, leaving, in time a population of white domestic ducks rearing white offspring. The initial control would involve removal of 70% of the present feral duck populations at Marina da Gama.
Fragments from report to City of Cape Town, City Planner’s Department. Compiled by Dr P.A.R. Hockey, June 1989
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