Aspects of population dynamics and feeding by piscivorous birds in the intermittently open Riet River estuary, Eastern Cape, South Africa

Abstract

Aspects of the population dynamics and feeding activity of piscivorous birds in the small (c. 5 ha) intermittently open Riet River estuary, on the south-eastern coastline of South Africa, were investigated monthly from August 2005 to July 2006. A total of 188 birds of 13 species were recorded, of which six were wading piscivores, four aerial divers and three were pursuit swimmers. The Reed Cormorant (Phalacrocorax africanus) was the numerically dominant species, with a mean of 8.25 (SD ± 7.90) individuals per count. Mean numbers of the Pied Kingfisher (Ceryle rudis) and Giant Kingfisher (Megaceryle maximus) were 3.42 (SD ± 1.20) and 1.17 (SD ± 0.60) individuals per count, respectively. The remaining 10 species revealed mean values <0.5 individuals per count. Breaching events were associated with a change in feeding groups from waders to pursuit feeders, and a decrease in total bird numbers, most likely due to loss of potential littoral zone foraging habitat for waders resulting from reduced water levels. The highest bird numbers were recorded in winter reflecting the migration of large numbers of Reed Cormorant into the system. Monthly food consumption by all piscivorous birds showed large temporal variability, ranging from 26.35 to 140.58 kg per month.

Keywords: avifauna, temporarily open/closed estuary

African Journal of Aquatic Science 2011, 36(1): 101–107