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Raptors And Reservoirs

16 October 2006

Raptors often fall into farm reservoirs and drown. A bird of prey with waterlogged feathers has little chance of getting out, especially if the dam is not full, since the sheer walls offer no purchase for it to scramble up.

There are records of 322 raptors of 29 species drowning in reservoirs in southern Africa (Anderson et al. 1999. Raptors drowning in farm reservoirs in South Africa. Ostrich 70(2): 139-144). This figure however probably only represents the tip of the iceberg, and it is likely that hundreds of raptors drown annually.

Conservationists are concerned as this unnecessary mortality results in raptors being lost from already small populations. In the arid areas of southern Africa, where natural ground water is scarce, and raptors have become dependent on artificial water sources, reservoir drownings account for a significant proportion of the unnatural mortality of birds of prey. In the Kalahari, for example, the Birds of Prey Working Group’s Abrie Maritz, recovered five Martial Eagles from farm reservoirs between 1991 and 1997, 8% of the population of these eagles in his study area. In another study in the Kgalagadi Transfrontier Park, Kotie Herholdt showed that drowning in water reservoirs accounted for 70% of all recorded raptor mortalities.

Drowned raptors also have implications for farmers. The carcass often floats in the water for several days before being removed, and as a result the water is polluted and often rendered unpotable for humans and livestock. These reservoirs then have to be emptied and cleaned.

For several years, ornithologists and conservationists have hypothesised about the reasons for the drownings, and three theories have been proposed to explain this unfortunate mortality factor:

– When perched on the wall to drink, raptors may accidentally slip into the reservoir.

– They may not be able to judge the depth of the water and therefore attempt to stand in the reservoir. This theory is supported by the many observations of raptors standing in the shallow water of rivers and dams while drinking and bathing. This may be important in hot climates as it would assist with thermoregulation.

– Birds of prey, especially vultures, which have eaten poison, such as strychnine (which apparently induces thirst), may plunge into the nearest water in an attempt to slake their thirst; and a drowning bird may attract others in what may appear to them to be a feeding melee.

The first two theories may explain why individual birds drown in reservoirs; while the third theory may explain the mass drownings of vultures. At least 13 vulture mass drowning incidents have been documented, including the mass drowning of 16 African White-backed Vultures in a reservoir near Vryburg (North West), 38 African White-backed Vultures in a reservoir near Vanzylsrus (Northern Cape), 64 Cape Vultures during four separate incidents in the same reservoir in the Eastern Cape, and, during April 2006, 13 Cape Vultures in a reservoir near Queenstown (Eastern Cape).

There are simple solutions to prevent reservoir drownings and these are being advocated by BoPWG and conservation authorities during extension programmes, especially in the arid parts of southern Africa. In order to promote awareness of the solutions and describe the mitigation measures, a pamphlet has been produced, articles have been published in farmers’ magazines, and the issue has featured on television and radio programmes. The proposed mitigation methods include:

- Keep the reservoir full, but this is not always feasible.

- Cover the reservoir with shade cloth or another material. This method has the added advantages of reducing evaporation and preventing the growth of algae.

- A more simply method is place a wooden plank, log, ladder or branch into the reservoir, attaching one end firmly to the side. A drowning bird will then be able to grasp onto the structure and lift itself out of the water.