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University of Cape Town (2015)

Martial eagles and the national power grid in South Africa : the implications of pylon-nesting for conservation management

Berndt, Jessie

Titre : Martial eagles and the national power grid in South Africa : the implications of pylon-nesting for conservation management

Auteur : Berndt, Jessie

Université de soutenance : University of Cape Town.

Grade : Master of Science 2015

Résumé partiel
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.


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