WILDLIFE MANAGEMENT- CONSTRUCTING DAMS FOR WATERFOWL
Constructing dams for waterfowl
D N Johnson
KwaZulu-Natal is not a naturally good area for waterfowl. The landscape is hilly and therefore there are few areas with open expanses of shallow water - the favourite habitat of waterfowl. Dam construction therefore increases the amount of habitat available.
 
However, the actual numbers and diversity of waterfowl attracted to a dam, depend greatly upon whether or not attention is paid to their particular habitat requirements.
Dam site and shape

Waterfowl like bare, open, level terrain, which provides large areas of shallow water. Here, they loaf and roost and find a substantial proportion of their food in the form of aquatic plants, floating seeds and small invertebrates.
The simplest way to create this habitat is to site a dam so that the land at, and above and below the high water mark is flat or gently sloping. If this is not possible, the same effect can be achieved with a bulldozer. In such a case, and indeed in any other earth moving operations, it is essential to spread the topsoil back evenly onto the ground once landscaping is complete. Topsoil is, however, superfluous at the deep end of the dam where nothing will grow, and it can be scraped up to the shallow end should it be required there.

If the terrain is such that no islands will be present in the dam, they can be created artificially. Islands can be any shape or size although large islands are better than small and several islands are better than one. The banks of the islands must be gently sloping, and the top of the island should only protrude a few centimetres out of the water. Steep earth banks rapidly become eroded by wave action, and ducks do not readily walk up steep slopes. Islands become particularly valuable refuges if shallow feeding areas adjoin them. This is because moulting flightless ducks can survive there. These feeding areas can easily be shaped whilst constructing the island. The productivity of feeding areas can be improved if a few loads of cow manure are dumped there.

Planting vegetation
Trees offer hiding places for predators and impede flight paths, so should never be planted on islands or anywhere else used by ducks. Islands can be planted with grass - not kikuyu, which is of little value to most wild life - or low-growing food plants, and the only other features that ducks will find attractive are a few boulders and dead logs, hollow if possible.

Reeds are an asset, provided they do not take over the main feeding areas. The best place to plant reeds is in isolated shallow spots from which they cannot "escape". Reeds attract bishop and widow birds, canaries, waxbills, warblers and roosting swallows as well as providing refuge for ducks. However, reeds can be strong invaders and any appearing in the main feeding areas should be uprooted.

Waterfowl often feed on land adjacent to the shore. They like young grass and other green material as well as seeds. This natural food supply can be improved by planting suitable crops, preferably on already disturbed ground rather than on natural veld. Buckwheat, millet, lotus, and grain sorghum are best and no particular care need be taken with the crop, which can be allowed to grow up with grass and weeds.

Aquatic vegetation tends to appear in the shallows after a time, but the process can be greatly speeded by planting. By far the most desirable plants are the various species of Potamogeton. Potamogeton grows wholly submerged in water and can be recognized by its soft stem reaching up to the water surface; its leaves, often narrow and grasslike, which float on the surface or are suspended below it, and its small whitish flowers. It can be planted by tying small stones to individual plants and dropping them into the water. Whilst anchored by the stones, the plants will soon root.

There are other aquatic plants of value, but care should be exercised before introducing them. The reason being is that some aquatics would prove disastrous. The three worst, Kariba weed, water hyacinth and parrot’s feather are very attractive in appearance but can totally choke a dam in a short time. For this reason they are proclaimed noxious weeds. Note that any aquatic plant can become a nuisance. It is often difficult to predict in advance whether a weed problem could arise. Much depends upon factors beyond the landowner’s control e.g. an inwashing of fertilizer from a neighbouring farm. If a weed problem arises, advice is available.

Water levels
In natural wetlands, water levels are rarely static and research has shown that these fluctuations improve the productivity of both plants and aquatic invertebrates. Lowering the water levels has several positive effects. Firstly, exposing bare mud improves the germination of many emergent aquatic plants, which would gradually disappear if the water level were static. Secondly, it enables temporary colonization by waterside weeds, which produce plenty of food. When killed by rising water, these weeds additionally provide natural manure. Thirdly, it profoundly alters the invertebrate community living in the mud. In permanently submerged mud there is a high diversity of invertebrates but total numbers are relatively low because the presence of specialized predatory invertebrates. A temporary drying out of the mud greatly reduces numbers initially, but upon re-flooding there is a great flush of prey species before the predators become re-established.

Temporarily raising the water level above normal also has positive effects. Shallow flooded grassland is a haven for waterfowl, and when the water level is again lowered, becomes ideal nesting habitat for plovers. A change of 1 or 2 m in water level, approximately once or twice a year, is sufficient to effect these improvements.

Disturbance
Waterfowl, particularly ducks, dislike regular disturbance. Accordingly, domestic stock must be excluded from prime areas, if necessary by fencing. Angling can be disturbing if regularly practiced at the shallow end of a dam. Angling debris, in the form of discarded hooks, lead weights and nylon line is very dangerous and contributes substantially to waterfowl mortality. Boating is also detrimental to waterfowl and is only practicable on very large dams with deepwater areas well removed from islands and shallows.

It is often thought that coots exclude ducks, the "proof" being that some dams support only coots. The truth is that many farm dams have little or no duck habitat but are very suitable for coots. Coots do not need shallow or flat land but are happy in 1 or 2m of water, where weedy growth thrives. They are an attractive and interesting addition to any dam.

Breeding
Ducks are very mobile and fussy in their choice of breeding habitat. Most species prefer temporary pans and many of KwaZulu-Natal’s ducks emigrate to the Mozambique plain, northern Botswana or Zambia to breed. This is why breeding success on farm dams seems low. Nevertheless, there are many ways of encouraging breeding locally. Some ducks will nest on the shore amongst dense grass, sometimes a long way from water. They require grass of the right age (1-2 years) for cover and some short grass pathways down to the water. Ideally, the grassy area around the shallows should consist of a mosaic of grass and food plants of different ages. Rotational burning or mowing may be necessary to provide such habitat.

Artificial nest sites have proved very successful in Europe and North America, but have yet to be tried in South Africa.A roofed nest box is simple to make and forms an acceptable substitute for a hollow log. An ideal size for a duck is 300 x 300 x 450mm high with an entrance hole of 150 x 150mm. The entrance must be towards the top of one of the side faces. Egyptian geese like a slightly bigger box. Waterfowl are much more likely to accept nest boxes if the inside is painted black and if wood shavings or dried leaves are placed inside. The floor must be provided with drainage holes. Finally, the box must be securely placed at least 1m above ground level, preferably on an island.

Ducks sometimes nest in inverted hollow cones if these are staked amongst emergent vegetation. An ideal cone has a diameter of 300mm and a depth of 250mm. Any material can be used but wire mesh is best. Once staked in position, the cone should be almost filled with cut grass.

Pitcher-shaped nest baskets have proved very successful in Europe. The basket should be 800mm long and have an entrance of 150mm in diameter. It must be mounted at a height of 2m on trestles so that its long axis is about 30o above the horizontal. Nest baskets can be mounted on islands or over the water.
Rafts provide nesting habitat and have the advantage of being immune to changes in the water level. A simple raft can be made from four hollow floats linked by an angle-iron frame. A wooden platform plus retaining walls can be fitted on top of this and 15cm of soil completes the structure. This floating island will support its own permanent vegetation. Anchors must be attached to opposite corners of the raft to prevent swinging in the wind.

Duckling survival
Duckling survival is often poor on farm dams and predators are usually blamed. Monitor lizards and mongoose are sometimes responsible, but so too are herons, fish eagles and even large bass and trout. However, the amount and quality of food available are more important factors. Ducklings need more protein, in the form of small invertebrates, than do adults. Recent research has shown that ducklings and small fish compete directly for the same food. Good duckling survival is associated with low fish stocking rates and visa versa. A solution to this problem is to encourage fish-eating birds, which anyway, are interesting in their own right. Cormorants and darters prefer fish of 40g and less and even the whitebreasted cormorant rarely takes anything above 100g. So they hardly threaten angling interests and in bass dams actually improve the angling by removing the tiddlers. Accordingly, leave a few emergent dead branches or stems on which they can perch. There are two other ways of improving the food supply for ducklings. One is to manipulate water levels. A drop in winter followed by flooding in August should cause an invertebrate flush. Another method is to spread straw in a shallow area. After a few days, it will sink to the bottom where it provides both food and cover for invertebrates and eventually enriches the mud.

Are waterfowl pests?
In some areas geese and yellowbills eat grain crops and can cause local damage. However, the scale is often exaggerated. A large male spurwig eats about 200g of maize per day, other waterfowl proportionately less. Even immediately around waterfowl roosts crop damage rarely exceeds 1%. If large numbers are attracted to crops they could be considered an additionals resource and shot for revenue and food.

Introductions
There is no need to introduce waterfowl to a dam. If it is attractive, they will quickly arrive of their own accord. In particular, it is a mistake to release alien species and such a release is only legal under permit. The reason is that several alien species are too aggressive and successful. The mallard for example, tends to exclude other species. It is also likely to interbreed with local species such as the yellowbill and this is highly undesirable.

KEY POINTS
Ensure that the dam has an extensive area of shallow water.
Fluctuating water levels improve food supplies.
Do not plant trees near areas used by waterfowl.
Waterfowl are sensitive to disturbances.
Artificial nest sites improve breeding success.
Release of captive-bred birds is unnecessary and undesirable.