A new understanding of the Yellow-billed Duck in South Africa

17 Oct 2007 - By Dr. Aldo Berruti

A new understanding of the Yellow-billed Duck "Anas undulata" in South Africa

The pivotal importance of maize in the ecology of the Yellow-billed Duck in South Africa has not been fully recognised. Whilst Viljoen (2005 - AGRED’s Gamebirds of South Africa) states that Yellow-billed Duck feed on maize in agricultural areas, the much-read summaries of Maclean (1997 - Atlas of Southern Africa Birds, Vol. 1) and Vernon & Dean (2005 - Roberts’ Birds of South Africa) do not even mention maize as a food.

The development of AGRED’s Vrede gamebird conservancy has provided a new understanding on the ecology of the Yellow-billed Duck. In general, ecological rulebooks say that the largest numbers of ducks occur on wetlands and dams with substantial shallows, and with much marginal and emergent vegetation, because such wetlands offer large amounts of food. This is usually true. Therefore, I was puzzled when I observed large numbers (up to 300) of Yellow-billed Duck on certain dams in the Vrede area. Most of these dams had bare shorelines, minimal shallow margins and no wetland vegetation. Why are there such large numbers of Yellow-billed Duck there?

Subsequently, two sets of observations in the Vrede area explained why Yellow-billed Duck occur in large numbers on apparently unsuitable dams. In July 2007, I was surveying a farm in the Vrede area in the company of the farm manager. I saw about 100 Yellow-billed Ducks on a dam no larger than 200m2 in area, and with bare steeply-sloping shorelines. Then another group of 30 Yellow-billed Ducks then flew up from dry land including cattle licks next to the dam. The manager told me that the ducks were eating whole maize kernels in the cowpats. On examination, I saw that these cowpats contained many whole maize kernels. The reason that the ducks were on the small “unsuitable” dam was because the dam provided access to a prime food source – the maize kernels in cowpats.

A second set of observations took place late in July 2007, when Nic Lowry and I were watching ducks movements at three dams, with observations beginning before sunrise to determine flight lines for shooting. We saw that the major movement of ducks came in the early morning when the Yellow-billed Ducks flew to harvested maize fields nearby to feed. Opportunistic observations throughout the day showed that the numbers of ducks remained more or less constant on at least two of these dams. The bare “unsuitable” dams which supported large numbers of Yellow-billed Duck were very close to maize fields.

How important is maize to Yellow-billed Duck? Maclean (1997 - The Atlas of Southern Africa) stated that the Yellow-billed Duck is the most common South African duck of the central plateau (Highveld), where maize is grown. It is probable that the Yellow-billed Duck feeding behaviour on maize in winter seen at Vrede is typical of the bird throughout its range in the Highveld.

Secondly, the maize is available for part of the year. Harvesting of maize takes place in May to July. Maize cobs or whole kernels are spilt onto the ground in the harvesting process and then become available to the ducks. However, this maize is eaten by many animal species through the winter and most remaining whole kernels are covered by ploughing which takes place from July to October. Because there is year-to-year variation in harvesting and ploughing dates, maize will be available on the ground for 3-4 months of the year.

It seems likely that maize is a critical food resource for the Yellow-billed Duck in winter, and its availability is a principal determinant of their abundance and distribution on a sub-continental and local scale. Because farmers also have created many thousands of small dams, they have created a habitat mosaic ideal for this species. This would suggest that intensive agriculture has probably vastly increased the numbers of Yellow-billed Ducks in South Africa. Maclean (1994) suggested that the Yellow-billed Duck has probably increased in historical times because of the creation of many small dams.

What are the implications for sustainable utilisation through shooting of the Yellow-billed Ducks? Estimation of the sustainable limits on harvesting of ducks is not simple, because of their great mobility. Thus, in sharp contrast to guineafowl, partridges and francolin, in which the numbers of birds in an area is a direct reflection of the management of that area, ducks may be shot thousands of kilometres away from they were bred. There have been concerns about perceived excessive harvesting of ducks in certain areas (e.g. as recently reported in the uplands of KwaZulu-Natal). The essence of the concern is that the species is threatened because ducks bred on natural wetlands are being overshot in peripheral “sink” areas.

Thus this insight offers a different scenario for understanding the threat of over-harvesting. Firstly, Yellow-billed Duck are probably far more abundant than in pre-agricultural times and are probably less vulnerable to over-harvesting than previously accepted because they are not as dependent on natural wetlands as thought. Secondly, Yellow-billed Ducks are widely distributed in maize-growing areas and breed throughout this region. Thus even if they are over-harvested in any one area, there is no compelling reason why Yellow-billed Ducks in unexploited areas with adequate food and water resources should move en masse into areas of over-exploitation.

The Egyptian Goose (and Spur-winged Goose to a lesser extent) has become much more common in recent years and has become an agricultural pest. Whilst the Yellow-billed Duck is not an agricultural pest, this new insight places the Yellow-billed Duck firmly in the category of those species that have benefited greatly from agriculture.

Nonetheless, AGRED strongly advocates a policy of sustainable (and legal) harvest of ducks including the Yellow-billed Duck to ensure their long-term persistence in high numbers everywhere. Such a policy is also integral to an overall harvesting strategy which is ethically, morally and ecologically defensible.

References

Maclean, G.L.M. 1997. The Yellow-billed Duck. In: Harrison, J.A., Allan, D.G., Underhill, L.G., Herremans, M., Tree, A.J., Parker, V. & Brown, C.J. (Eds). 1997. The Atlas of Southern African birds. Vol. 1: Non-passerines. BirdLife South Africa, Johannesburg.

Vernon, C.J. & Dean, W.R.J. 2005. The Yellow-billed Duck. In: Hockey, P.A.R., Dean, W.R.J. & Ryan, P.G. (eds). 2005. Roberts – Birds of Southern Africa, VIIth edition. The Trustees of the John Voelcker Bird Book Fund, Cape Town.

Viljoen, P.J. 2005. AGRED’s Gamebirds of Southern Africa Field Identification and management. AGRED, Johannesburg.