flood impact on wetland birds

One of the features of a warming planet is increasingly erratic rainfall; years of drought followed by devastating floods. Fortunately, many waterbirds are pre-adapted to cope with such extremes, especially in southern Africa where they have evolved to exploit episodic rainfall events in semi-arid and arid regions. But how do waterbirds respond to floods in areas where rainfall - and access to water - is more predictable? Peter Ryan explores the consequences of recent floods on the birds of the Western Cape's Olifants River valley.

- PETER RYAN

the maxim ‘nature abhors a vacuum’ reflects the fact that evolution favours strategies to take advantage of new opportunities. Most organisms respond to favourable conditions by breeding, but the rate of population growth is limited by the number of offspring per breeding attempt and the generation time. For birds, these are generally conservative relative to many other animals. Where birds have an advantage is through their great mobility, which en ables them to exploit short-term food resources through movement.

Waterbirds are past masters of this strategy, moving to areas where conditions are suitable and breeding when they get there. Any birder with a wetland in their local patch will have seen how the numbers of waterbirds fluctuate due to local immigration and emigration. The reasons for such movements are more difficult to infer, especially if your perspective is limited to a single site. Numbers might increase because conditions have become more favourable, attracting birds from surrounding areas. Alternatively, conditions might have deteriorated elsewhere, forcing birds to move to new areas. For example, numbers of ducks at Strandfontein Sewage Works in Cape Town peak in late summer, when seasonal wetlands in this winter-rainfall area dry out.

ABOVE Black-necked Grebe was one of the eight new wetland bird species I recorded in the Olifants River valley after the June 2023 flood.

ABOVE In June 2023, motorists attempting to cross the Olifants River had more than potholes to contend with!

ABOVE, RIGHT Most wetland birds in the valley occur on farm dams rather than along the river. Many of these were fuller than normal, losing their emergent vegetation.