Causes and consequences of spatial variation in sex ratios in a declining bird species

Morrison, Catriona A., Robinson, Robert A., Clark, Jacquie A. and Gill, Jennifer A. (2016) Causes and consequences of spatial variation in sex ratios in a declining bird species. Journal of Animal Ecology, 85 (5). 1298–1306. ISSN 0021-8790

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      Abstract

      1.Male-biased sex ratios occur in many bird species, particularly in those with small or declining populations, but the causes of these skews and their consequences for local population demography are rarely known. Within-species variation in sex ratios can help to identify the demographic and behavioural processes associated with such biases. 2.Small populations may be more likely to have skewed sex ratios if sex differences in survival, recruitment or dispersal vary with local abundance. Analyses of species with highly variable local abundances can help to identify these mechanisms and the implications for spatial variation in demography. Many migratory bird species are currently undergoing rapid and severe declines in abundance in parts of their breeding ranges, and thus have sufficient spatial variation in abundance to explore the extent of sex ratio biases, their causes and implications. 3.Using national-scale bird ringing data for one such species (willow warbler, Phylloscopus trochilus), we show that sex ratios vary greatly across Britain, and that male-biased sites are more frequent in areas of low abundance, which are now widespread across much of south and east England. These sex ratio biases are sufficient to impact local productivity, as the relative number of juveniles caught at survey sites declines significantly with increasing sex ratio skew. 4.Sex differences in survival could influence this sex ratio variation, but we find little evidence for sex differences in survival increasing with sex ratio skew. In addition, sex ratios have become male-biased over the last two decades but there are no such trends in adult survival rates for males or females. This suggests that lower female recruitment into low abundance sites is contributing to these skews. 5.These findings suggest that male-biased sex ratios in small and declining populations can arise through local-scale sex-differences in survival and dispersal, with females recruiting disproportionately into larger populations. Given the high level of spatial variation in population declines and abundance of many migratory bird species across Europe at present, male-biased small populations may be increasingly common. As singing males are the primary records used in surveys of these species, and as unpaired males often sing throughout the breeding season, local sex ratio biases could also be masking the true extent of these population declines.

      Item Type: Article
      Additional Information: © 2016 The Authors. Journal of Animal Ecology published by John Wiley & Sons Ltd on behalf of British Ecological Society. This is an open access article under the terms of the Creative Commons Attribution License,which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
      Faculty \ School: Faculty of Science > School of Biological Sciences
      University of East Anglia > Faculty of Science > Research Centres > Centre for Ecology, Evolution and Conservation
      Depositing User: Pure Connector
      Date Deposited: 25 May 2016 13:00
      Last Modified: 28 Jun 2017 06:04
      URI: https://ueaeprints.uea.ac.uk/id/eprint/59050
      DOI: 10.1111/1365-2656.12556

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