Aspects of the behaviour and ecology of White-fronted Sandplovers and Sanderlings on a South African sandy beach

The dissertation comprises a general introduction, five chapters and a summary-synthesis which is designed to link the chapters into a cohesive account. Chapter 1 deals with the avian population of a southwestern Cape sandy shore and compares this population with those of sandy beaches in the eastern Cape and Natal. The abundance of birds and the estimated energy requirements of the avian populations in these three geographical areas increase during the austral summer, due to an influx of migrants from the northern hemisphere. The proportion of migrants to residents varies geographically, being greater in the southwestern Cape and Natal than in the eastern Cape. Chapter 2 summarizes information on the systematics, global distribution and morphology of two small shorebirds, the White-fronted Sandplover Charadrius marginatus and the Sanderling Calidris alba, which are identified in Chapter l as being dominant components of the southwestern Cape sandy shore avian population. Chapter 3 reports on the spatial and temporal use of the intertidal habitat by foraging White-fronted Sandplovers ·and Sanderlings. Interactions between these two species are discussed in the light of competition theory. The foraging ecology of the Sanderling and White-fronted Sandplover at Ouskip differed strikingly. Although both species exhibited a tidal cycle in foraging activities, rather than a diurnal cycle, White-fronted Sandplovers tended to feed during the latter half (i.e. mid to high tide), whereas Sanderlings fed during the first half (i.e. low to mid tide) of the tidal cycle. Both species' foraging activity also varied spatially and seasonally. White-fronted Sandplovers fed primarily in drier microhabitats above high tide level, and Sanderlings in wetter microhabitats near the water's edge. In Chapter 4, the size and structure of Sanderling flocks are described. The frequency distribution of flock sizes of wintering Sanderlings foraging on the sandy beach at Ouskip was bimodal, with a peak at small flock sizes and another at large flock sizes. It is hypothesized that large groups of Sanderlings consist of individuals searching for subterranean prey, and that small groups consist of subsets of these larger groups which have located food patches. Chapter 5 is an analysis of breeding periodicity in White-fronted Sandplovers in southern Africa, and focuses on the relationships between geographical and year-toyear variation in breeding activity and rainfall patterns, food availability and the possible effects of competition with migrant shorebirds. This species exhibits significant geographical variation in breeding season, and significant year-to-year variation within an area which has winter (May - August) rainfall. There was no year-to-year variation in breeding season within an area with summer (November - February) rainfall