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Long-term population declines of Palearctic passerine migrant birds : a signal from the Sahel ?

Bruno A. Walther, Adriaan Van Niekerk, Carsten Rahbek
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Conference Proceedings
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During the last decade, a database on the African distributions of Western Palearctic migrants has been collated which currently contains just over 250 000 point-locality records, mostly of passerine migrants (http://macroecology.ku.dk/resources/data_resources/african_migrants/). Research using this database has led to an improved understanding of the migrants’ distributions, especially of threatened species such as the Corncrake Crex crex, the Aquatic Warbler Acrocephalus paludicola, the Basra Reed Warbler Acrocephalus griseldis, the Cinereous Bunting Emberiza cineracea and the Ortolan Bunting Emberiza hortulana, as well as the identification of habitat use, macroecology and conservation priority areas. In this study, we used the available point-locality records to map the sub-Saharan distribution of 65 species of passerine migrants using a combination of presence-only and presence-absence distribution modelling techniques (Walther et al. 2010). Combining these distributions with data on the conservation status and population declines of 64 of these 65 species published by BirdLife International, we found that species which declined during the period 1970– 2000 were concentrated in the Sahelian region, while species with more stable populations were overwintering all across Africa. This clear geographical signal then led to a comprehensive literature review of recent climatological, biological and ecological changes in the Sahelian and more southerly Sudanian zone. The main conclusions are: (1) the 1968–1997 drought was exceptionally severe; (2) year-to-year climatic variability was very high in the Sahel, and higher than in the rest of sub-Saharan Africa; (3) these climate extremes had unusually strong repercussions on soil erosion, above- and below-ground water levels, and natural vegetation, especially perennial woody vegetation, with some negative effects still apparent to this day despite the recent re-greening of the Sahel; (4) driven by one of the highest human population growth rates in the world, agricultural expansion and loss of natural non-forest vegetation was higher in the Sahelian and Sudanian zones than in any other sub-Saharan region; (5) the effects of cattle grazing and wood exploitation may also have been more severe in this region than in other sub-Saharan regions. The implications for migrant birds are that, even before the drought beginning in the late 1960s, there was a long-term trend of humans appropriating more and more natural resources, recently accelerated through exponential population growth and more intensive agricultural methods. Therefore, fewer and fewer natural resources are being left for animal populations, including migrant birds. The recent re-greening of the Sahel and the parallel improvements in agricultural techniques during the last decade have improved the welfare of human populations, but have had no or very little positiveeffect on natural resources utilized by wild animal populations. This widespread ecological transformation, which was apparently more severe in the Sahelian region than in any other sub- Saharan region, may therefore be one of the main long-term drivers of the population declines of those migrants which mainly overwinter in these regions, and possibly even for bird species which use these regions only for refuelling. Discussing the results from this and other recent studies, we finish by making a few recommendations for future research and conservation work.

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