Browsing by Author "Siegfried, Roy"

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    Aspects of plant dispersal in the southwestern Cape with particular reference to the roles of birds as dispersal agents
    (1988) Knight, Richard Spencer; Knight, Richard Spencer; Siegfried, Roy; Moll, Eugene J
    The dispersal of plants with particular attention to the roles birds play as dispersal agents was studied in the southwestern Cape, South Africa from 1983 to 1985. The research was organized as ten inter-related studies, each with an independent data base. Each chapter focused on a different scale of plant dispersal processes ranging from regional assemblages of plant species to individual species. At the largest scale, the seven vegetation types commonly occurring in the southwestern Cape were examined for incidence of vertebrate-dispersed plants. Coastal Thicket and Afromontane Forest were found to be the richest in these species. The colonization of vertebrate-dispersed plants was examined in an artificially cleared area of Mountain Fynbos vegetation. Enhanced densities of vertebrate-dispersed species were found in areas where perches had been provided. The seasonal availability of vertebrate-dispersed species was found to be most continuous in Coastal Thicket vegetation. Fruit displays of vertebrate-dispersed indigenous plants were found to vary from those that were sporadic and inconspicuous, to those that were conspicuous and predictable, whereas those of alien plants were usually large and conspicuous. Avian use of these fleshy fruits in Coastal Thicket was examined and found to be proportional to their availability. A study of fruit presentation in relation to leaf number and stem thickness suggested that sessile, stem attached fruits have fewer local leaves at time of ripening than fruits which are presented in panicles. This may enhance accessibility and conspicuousness of stem attached fruits for dispersal by birds. Four autecological studies tested certain predictions arising from models developed to describe fruit/frugivore interactions. The dispersal of the alien Acacia cyclops seeds by the indigenous Black Korhaan Eupodotis afra suggested that successful fruit/frugivore relationships are not necessarily the product of reciprocal evolution. The study on Chrysanthemoides monilifera found that efficient dispersal systems are not limited to plants producing small quantities of lipid-rich fruits and to dispersal by obligate frugivores. The abiotic dispersal of Quercus robur was found to be efficient in relation to vertebrate-dispersal. The study on Protasparagus aethiopicus found that the morphology of fleshy fruits may also reflect attempts to overcome the effects of non-dispersing seed predators. 1It is concluded that a gradient from a predictable to unpredictable fruit availability provides a better basis for studying fruit/frugivore interactions than the coevolutionary models. previously presented.
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    Aspects of the behaviour and ecology of White-fronted Sandplovers and Sanderlings on a South African sandy beach
    (1986) Crowe, Anna Aletta; Siegfried, Roy
    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.
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    Aspects of the foraging and breeding ecology of the Southern African Kestrel, Falco tinnunculus rupicolus
    (1993) Van Zyl, Anthony John; Siegfried, Roy
    A study of the ecology of the Southern African Kestrel was made in two areas of differing topography in the Eastern Cape Province (32 °S) of South Africa. The main objectives of the study were to describe foraging parameters (behaviour-time budgets, hunting success and diet) and breeding parameters (clutch size, hatching and fledging success, nestling, fledging and post-fledging periods) of the Southern African Kestrel. These parameters were compared to those from studies made on the European Kestrel (Falco tinnunculus tinnunculus) in western Europe (53 °N) in an environment which fluctuates relatively more than the South African study sites. Predictions concerning foraging and breeding ecology made by life-history theory in stable and fluctuating environments prompted the erection of the following hypotheses which were examined in this study. 1. Common Kestrels (Falco tinnunculus) in relatively stable environments have a more consistent but lower foraging yield than those breeding in fluctuating environments. 2. Common Kestrels in relatively stable environments have lower reproductive rates (longer incubation, nestling and post-fledgling dependence periods, later age at first breeding and longer life-span) than those in fluctuating environments. Southern African Kestrels spent different proportions of time of the two main hunting methods used, perch- and hover-hunting, than the European Kestrel. Perch-hunting was the predominant hunting method used by the Southern African Kestrel compared to hover-hunting in the European Kestrel. Southern African Kestrels had a higher prey-capture success for both hunting techniques. However, daily food intake was lower in Southern African Kestrels because a high proportion of the diet consisted of small invertebrates. Average clutch size was smaller in the Southern African Kestrel than that of the European Kestrel and Southern African Kestrels fledged on average fewer young. However, incubation, nestling and post-fledging periods were within the range recorded for the European Kestrel. The hypothesis that Common Kestrels in relatively stable environments have lower foraging yields is supported by this study. Although results from this study suggest that food yields are more consistent in stable environments, it was unable to evaluate this adequately. The hypothesis that reproductive rates are lower in relatively stable environments is partly supported in this study, but more data are required for parameters such as incubation and nestling periods to confirm such trends.
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    Breeding Jackass Penguins as pelagic predators
    (1986) Wilson, Rory Paul; Siegfried, Roy
    The foraging of breeding Jackass Penguins Spheniscus demersus was studied in and around southwestern Cape Province, Saldanha Bay (33⁰ S, 18⁰ E), South Africa. Penguins are difficult to observe at sea. Hence, I devised a number of new techniques for studying the foraging behaviour of Jackass Penguins at sea. I built electronic and autoradiographic remote-sensing devices to measure swimming speed, distance travelled and time spent at each depth by foraging Jackass Penguins. Penguin swimming speed was reduced in proportion to the cross-sectional area of the devices, and results derived from birds wearing the devices had to be interpreted accordingly. Penguins do not regurgitate their stomach contents when handled, so I constructed a wet-offloading stomach pump which extracted 100% of the stomach contents. Using this pump, I determined that the rate of digestion of fish and squid by Jackass Penguins differed. Care is needed in diet interpretations where both fish and squid are major food items.
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    Dispersion of seabirds at sea in the Southern Ocean
    (1982) Griffiths, Andrew Martin; Siegfried, Roy
    The feasibility of obtaining information on the dispersion of seabirds at sea precise enough to reflect changes in their prey was investigated. A standardized technique for counting birds from a moving ship, designed to limit biases due to birds circling, following and/or deviating towards/from the ship, is suggested. An interspecific comparison of 31 seabird species was made to determine which species yielded the most accurate censuses. Although many species are attracted towards the ship, only the Wandering Albatross Diomedea exulans follows for long periods. Counts from a stationary ship are shown to be unsuitable for abundance and biomass estimates, because of the accumulation of birds around the ship. The avifauna at sea is described in terms of species richness, diversity, abundance, biomass and trophic groups of 42 pelagic species (penguins excluded). Birds eating plankton and cephalopods are the most abundant; few birds eat fish. Plankton- and cephalopod-eaters occur most abundantly in the south and north of the study area, respectively. An association between their distribution and the availability of their principal prey is proposed. The effect of five abiotic features on seabird distribution was investigated. Although significant preference for specific ranges of features is demonstrated, linear correlations are weak (maximum correlation coefficient (r = 0.325). Abiotic features associated with the distribution of the Snow Petrel Pagodroma nivea and the Antarctic Petrel Thalassoica antarctica were investigated in greater detail. Statistical relationships between the species' occurrence and measured oceanographic and meteorological features are inconclusive. Associations with prey are discounted, because of the birds' apparently unspecialized diet and opportunistic feeding. The two species occur in or near sea-ice. Their restriction to this area and the concomitant absence of other procellariiform species appears to be consequent on the species' flight characteristics. The merits of using seabirds at sea as biological indicators of prey resources are discussed.
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    Feeding biology and energetics of King Aptenodytes patagonicus and Gentoo Pygoscelis papua Penguins at sub-Antarctic Marion Island
    (1990) Adams, Nigel John; Siegfried, Roy
    Adaptations of penguins for pursuit diving have resulted in reduced foraging ranges, compared to volant seabirds. However, their classification as inshore, offshore or pelagic foragers, based on rates of food provisioning to chicks, still has merit as a framework around which aspects of their feeding biology and allocation of energy within individual energy budgets may be discussed. The average rates at which seabird species supply food to chicks may influence many aspects of their biology. I defined some of the foraging parameters of the pelagic feeding King Penguin Aptenodytes patagonicus and the inshore feeding Gentoo Penguin Pygoscelis papua at sub-Antarctic Marion Island.
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    The incidence and effects of ingested plastic in seabirds
    (1986) Ryan, Peter G; Siegfried, Roy
    This thesis comprises two major sections: an assessment of the incidence of plastic particles at sea and in seabirds {Chapters 1-3), and an investigation of the postulated effects of plastic ingestion on seabirds (Chapters 4-7). The incidence of plastic at sea was recorded from neuston trawls performed monthly at 120 sampling stations off the southwestern Cape, South Africa, during 1977-78. The types of plastic particles collected are described, and the temporal and spatial distribution patterns of plastic pollution are discussed. Ingested plastic was recorded from 36 of 60 seabird species sampled. The effect of different sampling techniques on the incidence of plastic is discussed. The frequencies of occurrence of plastic colour-types in birds are compared with those of particles collected at sea in neuston trawls. Pale particles were under-represented in all species, but the disparity was less for small than for large species, which may account for the higher incidence of ingested plastic in small than in large species.
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    Where birds are rare or fill the air : the protection of the endemic and the nomadic avifaunas of the Karoo
    (1995) Dean, William Richard John; Siegfried, Roy
    The primary objectives of this study were to assess the protected status of birds in the Succulent and Nama-Karoo, and to identify areas, using a GIS approach, where new protected areas could be established. The Karoo sensu lato lacks a distinctive avifauna, but is rich in species. A total of 407 bird species has been recorded in the Succulent and Nama-Karoo, of which 294 species are considered typical of the region. The Karoo is not particularly rich in bird species endemic to the region, and all Karoo endemic species occur in both biomes. The Nama-Karoo has an unusually high species richness of nomadic birds, and both the Succulent and Nama-Karoo have an unusually high species richness of larks (Alaudidae) compared with other biomes. The Succulent and Nama-Karoo have stochastically low annual rainfall and generally low above-ground primary production. Rainfall is often highly localized and varies annually in amount and timing. The variability of the rainfall and long dry spells or severe droughts create a mosaic so that high and low resource areas occur as patches in the landscape. Resident species of birds tend to maintain low densities and wait for rainfall events, whereas nomadic species search for high resource patches scattered in time and space, so that their respective densities likewise vary temporally and spatially. Compared with other biomes in southern Africa, there is a high species richness of nomadic birds in the Karoo. The small nomadic larks are most common in areas of perennial and annual desert grasses, and they feed on grass seeds, whereas resident granivorous birds tend to feed on the seeds of forbs and shrubs. Clutch sizes of the nomadic larks birds are small, apparently selected for by the need for a short breeding cycle.