Includes bibliographical references / Many large, sparsely distributed raptors are threatened by a host of anthropogenic factors, while a minority may actually benefit from some aspects of development and environmental change. Clarity on the size and trajectory of such populations is essential for effective conservation management, but can be difficult to achieve. One solution is to use multivariate habitat association models to derive critical estimates of distribution and abundance. The South African population of Martial Eagle Polemaetus bellicosus is currently estimated at < 800 adult birds , with the bulk of the known population believed to be residing in the larger protected areas. However, Martial Eagles also build nests on pylons that support high voltage transmission lines running through the largely treeless, semiarid landscapes of the Karoo. The main aim of this study was to develop a better understanding of the environmental factors that influence Martial Eagle territory densities and locations along South African transmission lines, and thereby estimate the size of this population and its relative importance to the national conservation status of this globally threatened species. I used habitat association models to d escribe Martial Eagle territory density in relation to eight environmental covariates. Models were first fitted to eagle territory data for the central Karoo regions, collected and pooled over the period 2002 - 2006, and then applied to predict the number of pairs present on each of three adjacent sections of unsurveyed line (northern, southern and eastern lines) . Once these model predictions were verified by a series of aerial and ground surveys, I fitted the models to all the known Martial Eagle territory records for the transmission network and extrapolated from these back to the rest of the network using the fitted relationships. Ultimately, the models predicted 52 additional Martial Eagle territories on the remaining transmission network with a confidence interval ranging from 38 to 67 (based on models that explained up to 39 % of the total variance in terms of only two explanatory terms – rainfall and the proportion of cultivated land). I then examined the role of territoriality and social structure in the eagle population in determining the location and dispersion of pylon nests. To do this I used the location of active nests from the original central Karoo data and a similar number of randomly selected points. I then asked whether I could predict the nest locations from each of the eight environmental covariates and distance to its nearest conspecific active nest or its nearest nest of any other large eagle species. Using a logistic generalised linear model with regression splines for distance to nearest other nest, I found that Martial Eagles strongly avoid proximity to conspecific nests (mean distance to conspecific nest = 28.2 km, range 2.5 - 167.1 km, n = 306). This result shows that minimum spacing should be considered in predicting the distribution of eagles on unsurveyed transmission lines. Lastly, I further investigated the geographical extent of pylon nesting in South African Martial Eagles, with particular focus on variation in the frequency of this behaviour in relation to biome - scale variation in the availability of trees as natural nest sites. To do this, I related Martial Eagle reporting rates generated by citizen - science bird atlas data to the density of transmission lines and biome types across South Africa. While these analyses yielded some suggestive results, such as significant positive and negative relationships between reporting rates and line density in the Desert (P = 0.02) versus the Savanna (P < 0.001) biomes respectively, data sparseness in arid areas and a generally low detection probability limited the conclusiveness of these results. The refined habitat association models developed in this study predict that the South African transmission grid supports 130 - 159 breeding pairs of Martial Eagle. This figure has never been estimated or calculated before, and suggests that 36 % of the national breeding population could reside largely in the commercial ranchland and nest on man - made structures. This result, which is at odds with the generally held belief that the Martial Eagle is increasingly confined to large protected areas, has significant implications for the thinking around the conservation management of this globally threatened species.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:uct/oai:localhost:11427/15583 |
| Date | January 2015 |
| Creators | Berndt, Jessie |
| Contributors | Jenkins, Andrew R, Altwegg, Res, Amar, Arjun |
| Publisher | University of Cape Town, Faculty of Science, Percy FitzPatrick Institute of African Ornithology |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Master Thesis, Masters, MSc |
| Format | application/pdf |
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