Long-term trends, seasonal abundance and energy consumption of waterbirds at Strandfontein, Western Cape, South Africa, 1953–1993

Abstract

Waterbird counts at Cope Flats Sewage Treatment Works (Strandfontein) near Cope Town are analysed for three periods: 1953–58, 1961–67 and 1983–93. Information on waterbird abundance is also provided for the period 1979–81. One hundred and one species of waterbirds were recorded. Resident birds predominated during summer and winter in all years. Lesser Flamingo, Phoenicopterus minor, and Red-knobbed Coot, Fulica cristata, were the most abundant species in the 1950s, Cape Teal, Anas capensis, and Greater Flamingo, Phoenicopterus ruber, in the 1960s, and Cape Cormorant, Phalacrocorax capensis, in the 1980s–90s The total avian biomass in the 1980s–90s was 62-fold higher compared with the 1950s, reflecting the increased abundance of many birds. In the 1980s–90s, the seasonal pattern of bird biomass paralleled that of bird abundance, reaching its peak in January and declining to its lowest value in October. The summer daily energy intake of the entire bird assemblage exceeded that of winter in all years except in 1980s–90s. The invertebrate feeders were the most important consumers in the 1980s–90s with the Greater Flamingo being responsible for the increased energy intake in late summer. The abundance of waterbirds increased progressively from the 1950s, reaching an average maximum of over 23 300 individuals in January during the 1980s–90s. Seventeen species were absent or recorded sporadically at Strandfontein in the 1950s compared with the 1980s–90s. Twenty-five species increased their abundance 10–90-fold, seven 100–600-fold, and one almost 2000-fold in the 1980s–90s compared with the 1950s Lesser Flamingo was the only species for which a considerable reduction in numbers was recorded. The increase in abundance of several species is attributed to changes in habitat availability, especially on increase in the water-surface area, The fluctuations in numbers between years is linked to changes in the sewage work system in the late 1970s, which altered water quality in the ponds and probably affected the food supply for some of the species. Strandfontein ranks fifth in terms of waterbird abundance compared with other South African wetlands and requires immediate conservation action.