Abundance and distribution of waterbirds on the Knysna Estuary

Abstract

Twelve waterbird counts were undertaken on the Knysna Estuary during January (summer) and July (winter) from 1993 to 1998. A total of 67 species were recorded; a median of 5343 waterbirds were present during summer, 76% of them Palearctic migrants, dropping to a median of 2336 waterbirds in winter. Waterbird numbers were about a third lower than normal during January 1997 following a flood in November 1996. There was a shift from a summer avifauna comprised predominantly of invertebrate-feeding migrant waders to a winter avifauna comprised mainly of piscivorous species. During the summer, 52% of the birds were Curlew Sandpipers and Grey Plovers, Greenshanks, Kelp Gulls and Whimbrels each made up more than 6% of the avifauna. During the winter Kelp Gulls, Cape and Reed Cormorants, Little Egrets and Sacred Ibises together comprised 62% of the avifauna. The majority of invertebrate-feeding birds were found on the intertidal mudbanks around the Ashmead Channel and on the Brenton side of the estuary below the rail bridge, whereas the shallow water areas between the rail and N2 bridges were important to piscivorous birds, especially during the winter. The Knysna Estuary, being the largest estuarine system in the warm temperate region of South Africa, supports the largest number of birds of any estuarine system between Cape Agulhas and Durban Bay. However, bird densities are not as high as on other important bird estuaries and it is hypothesised that this may be due to recreational disturbance and to the relatively low density and availability of macrobenthic invertebrates.