Attempts to explain the orders-of-magnitude variation observed in animal population sizes have principally focused on intrinsic differences between the taxa compared, but with limited success: most variation remains unexplained by such studies. However, animal population sizes may also vary in response to extrinsic factors, such as the environment occupied or the influence of human activities. Here, we use new estimates of the global population sizes of threatened bird species to examine extrinsic correlates of variation in their numbers, using general linear modelling and methods to control for phylogenetic relatedness. Threatened bird population sizes varied significantly with several extrinsic factors, including altitude, biogeographical region inhabited, type of extinction threat faced, and habitat used. They also vary with geographical range size, which was included in the analysis to control for its potentially confounding effects on the results. Details of the observed relationships, which vary with analytical method, are discussed. However, apart from geographical range size, none of the extrinsic variables analysed here explain more than a small percentage of the variation in threatened bird population sizes. Thus, it seems likely that a comprehensive explanation for why some species are common while others are rare will not be dominated by a single factor.
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