DISTRIBUTION OF SHOREBIRDS AT THE BERG RIVER ESTUARY, SOUTH-AFRICA, IN RELATION TO FORAGING MODE, FOOD-SUPPLY AND ENVIRONMENTAL FEATURES

Foraging densities of Curlew Sandpipers Calidris ferruginea and Grey Plovers Pluvialis squatarola at the Berg River estuary, South Africa, are examined in relation to prey and habitat attributes. There is a substantial overlap in the prey species eaten by Curlew Sandpipers and Grey Plovers, but the distribution patterns of the two species within the estuary could not be explained by the same prey attributes. The distribution of tactilely foraging Curlew Sandpipers was determined largely by the numerical abundance of nereid worms. They foraged preferentially on muddy rather than sandy substrata, but their distribution could not be explained on the basis of substratum penetrability. The density of visually foraging Grey Plovers was correlated with the biomass, rather than abundance, of the larger of the two common nereids on the estuary. The favoured foraging area of Grey Plovers had a high vegetation cover, but their foraging dispersion was not influenced by substratum type. The energy intake rates of Curlew Sandpipers were broadly inversely related to their foraging density, although the rate of prey capture was maximal at high prey density. These birds may use neighbour success rates in selecting foraging sites. Energy intake rates of Grey Plovers were very consistent across the estuary, suggesting that individuals occupy or defend areas which provide access to comparable food resources.