Browsing by Author "Du Plessis, Morné"

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    Chemical defence in the red-billed wood hoopoe : phoeniculus purpureus
    (2001) Law-Brown, Janette; Du Plessis, Morné
    Red-billed Woodhoopoes, Phoeniculus purpureus, produce a pungent smelling secretion from their uropygial gland. Previous researchers have noted this odour and there has been much speculation on its function. This encouraged me to undertake this study to determine the origin of the odour and the role that the secretion plays. The chemical analysis of this secretion shows that it consists of 17 compounds including acids, aldehydes, lactones and other miscellaneous compounds. Cultures of the secretion showed the presence of a symbiotic bacterium resident within the gland. Antibiotic treatment of the gland suggested that this bacterium was instrumental in the synthesis of the secretion of P. purpureus. This bacterium has not previously been identified and has been proposed as 'Enterococcus phoeniculicola' (GenBank accession number: AYO28437). The secretion was synthesized and the responses of a range of 13 pathogenic and one parasitic bacteria were tested against it and its individual constituent compounds. The synthetic secretion and seven of its constituent compounds inhibited the growth of these bacteria. However, exposing feather mites resident upon the birds to the synthetic secretion did not illicit any significant response from these mites. Wanless (1998) showed that the synthetic secretion causes an averse reaction in cats. In this study I was able to identify indole, phenol and propionaldehyde as being responsible for eliciting this reaction in cats. A similar averse reaction is shown by Rock Monitor Lizards, Varanus albigularis, upon encountering mice tainted with the synthetic secretion. This is the first demonstration of a symbiotic relationship between birds and bacteria in which the modification of the birds' uropygial oils affords the birds some protection against pathogens and preators.
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    Going through the motions : the impacts of frequent fires and grazing pressure on reproduction of Montane grassland birds
    (2002) Muchai, Samuel Muchane; Du Plessis, Morné
    Land management practices have been implicated as a cause for the decline of many grassland-nesting birds across the globe. While this effect has motivated extensive research and conservation in the developed northern hemisphere, it remains poorly addressed in southern hemisphere. Between 1998 and 2001 I examined the effect of fire frequency and grazing intensity on the density, breeding biology and nesting success of several grassland bird species that breed in the high altitude grasslands of Wakkerstroom, South Africa. I established study plots in heavily grazed and annually burned, lightly grazed and annually burned, and lightly grazed and biennially burned plots. These represent the most frequently used grassland management regimes in this region. Independent analysis of all study species showed that daily nesting success was higher under light than heavy grazing. Similarly, nesting success was higher under biennial burning when compared to annual burning. Nest predation was the major cause of nest failure during all three years for all species. The likelihood of nest predation clearly depended upon foliage in the immediate vicinity of the nest and vegetation within 10 m radius of the nest (the nest patch). In general, variables important in discriminating between successful and depredated nests across all species were directly related to vegetation cover, density and horizontal heterogeneity. I also examined micro-habitat choices, the form of natural selection and the adaptiveness of preferences in four coexisting grassland bird species (Yellow-breasted Pipit Hemimacronyx chloris, Grassveld Pipit Anthus cinamomerous, Orange-throated Longclaw Macrony capensis, and Ayre's Cisticola Cisticola ayressii). Breeding birds selected nest patches non-randomly and this differed between species. Comparison of vegetation features at successful and unsuccessful nests supported the idea that nesting success is a strong selective force on habitat choice. Nest success was higher in preferred than non-preferred habitat for all the four species, suggesting that preferences were adaptive. Estimation of fitness functions relating fitness of individuals to critical habitat features suggests that natural selection might favour preferences for specific habitat features. Although food abundance and thus amount of food available to populations of breeding birds significantly differed between management regimes, the study yielded no evidence for an effect of management-mediated food abundance on feeding rate, nestling provisioning rates, nestling growth rates, body condition, nest attentiveness and brooding effort. My results suggest that the food availability alone may not be the most important factor influencing the production of offspring. Instead, nest predation appeared to be of major importance in this system. Theoretically, nest predation increases with activity at nests, and predation rates should peak during the nestling stage when birds are feeding young. I tested this hypothesis using three ecologically similar grassland bird species (Yellow-breasted Pipit, Orange-throated Longclaw, and Grassveld Pipit). Parental activity was indeed greater during the nestling than incubation stage. Nest predation, however, did not increase with parental activity between these stages in all three study species. I conducted an experiment that controlled for parental activity (by reusing natural nests of the study species with artificial clutches) in order to test for nest-site effects. Nests that had a high risk of predation when used by active parents had a correspondingly high risk of predation when the same nests were re-used with artificial clutches (i.e. when controlling for parental activity). This result supports the notion that variation in nest-site quality often affects nest predation risk, and such effects could mask parental activity effects on nest predation. Once-nest site effects were accounted for, nest predation showed a positive increase with parental activity during the nestling stage within and across species. Collectively, the chapters of this thesis help to diagnose the causes and underlying mechanisms of grassland bird population decreases, and help to identify the most effective conservation actions. In short, conservation effort for grassland species should be directed at ensuring that their preferred critical nesting habitat is managed appropriately. I propose that current intensive grazing pressure and periodic burning should be relaxed by reducing stocking rates and burning less frequently to benefit grassland bird species. Ideally, grasslands should be burned biennially and grazed moderately.
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    Life-history evolution and cooperative breeding in the sociable weaver
    (2002) Covas-Monteiro, Rita; Du Plessis, Morné
    When compared to northern temperate counterparts, birds in the tropics and Southern Hemisphere regions, are characterised by low fecundity, long developmental periods and high Survival. This pattern has been attributed to hypotheses relating to food limitation, nest predation, and reduced seasonality leading to high adult survival. Still, to date few studies have investigated this issue. In particular, detailed studies from southern regions are scarce and as a result our understanding of this question remains weak. Another characteristic of 'southern' birds is a higher frequency of cooperatively-breeding species. It has previously been shown that cooperative breeding is most frequent amongst species with high survival and low fecundity. An explanation for this could be that long occupancy of territories by long-lived breeders leads to reduced opportunities of independent breeding by young birds. However, the biology of some species is such, that they do not experience shortage of breeding territories or other constraints, while still showing delayed dispersal and cooperative breeding. This study investigates what factors drive life-history evolution in a cooperatively breeding southern African passerine, the sociable weaver Philetairus socius. This species departs from the traditional cooperative breeding model in being a non-territorial colonial nester that inhabits an unpredictable semi-arid environment. Sociable weavers face no obvious constraints on independent reproduction, yet cooperative breeding is common. Therefore, I also aimed at determining what factors prompt delayed reproduction in this species, since this life history trait is the first step leading to cooperative breeding.