Browsing by Author "Kalejta, Bozena"

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    Aspects of the ecology of migrant shorebirds (Aves: Charadrii) at the Berg River estuary, South Africa
    (1992) Kalejta, Bozena; Hockey, Phil A R
    The waterbird populations of the Berg River estuary, South Africa, were studied from September 1987 to April 1989. The main objectives of the study were to assess the importance of the estuary for Palearctic-breeding migratory waders and to identify the factors affecting their distribution, abundance and behavioural patterns. The findings were related to the dispersion patterns of waders on the east Atlantic seaboard, and their significance was evaluated in the light of predictions of current models for the migratory behaviour of shorebirds. Controversy over the Winter competition model is discussed. The intertidal mudflats (144 ha) at the Berg River estuary support an exceptionally high density of migratory shorebirds during the austral summer. The high density of birds results in a high predation pressure. Although the rate of prey removal by birds is one of the highest recorded in the east Atlantic, it represents only 17% of the annual production of invertebrates. A bird-exclusion experiment confirmed that birds have little impact on their prey populations. Although the energy balance for the majority of species was apparently negative for much of the austral summer, the energy deficits measured in the field were less than those recorded in north temperate estuaries. The highest predation pressure by birds coincided with the period of highest production of the most important invertebrate prey. Because of low prey diversity in the estuary, there was considerable overlap in the prey species taken by different bird species. The preferred prey of most species were nereid worms. Within this prey class there were clear differences in the size classes of worms eaten by different birds. The distribution of birds on the estuary was related to both biotic and abiotic factors. The observed patterns of partitioning of both food and space resources was attributed to the superabundance of prey and to interspecific differences in foraging techniques unrelated to competition. The spatial redistribution of birds within the estuary during the first four months after their arrival from the breeding grounds was related to changes in feeding conditions on their preferred foraging sites, rather than density-dependent factors mediated by competition. I propose that a combination of high prey abundance and production, coupled with an apparently low level of competition, makes the Berg River an attractive nonbreeding site for migrant waders. Their relatively sound energy budgets and the benign climate may enhance nonbreeding survivorship sufficiently to offset any additional risks incurred in long-distance migration.