Going through the motions : the impacts of frequent fires and grazing pressure on reproduction of Montane grassland birds

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 200 I 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 microhabitat choices, the form of natural selection and the adaptiveness of preferences in four coexisting grassland bird species (YeHow-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 v Un ive rsi ty of Ca pe To wn 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, Orangethroated 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 (Le. 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. VI Un iv rsi ty of Ca pe To wn