Biased adult sex ratios in western Europe populations of little bustard Tetrax tetrax as a potential warning signal of unbalanced mortalities

Abstract

<p>Adult sex ratios (ASRs) have proved to correlate with population trends, which make them</p><p>potential useful indicators of a species’ population trajectory and conservation status. We</p><p>analysed ASRs and proportion of juveniles in flocks of an endangered steppe bird, the Little</p><p>Bustard Tetrax tetrax, using surveys made during the non-breeding period in seven areas within</p><p>its Western European range (one in Portugal, four in Spain, and two in France). We found</p><p>overall male-biased ASRs, as all the seven surveyed areas showed a male-biased ASR mean value.</p><p>Five areas were below the threshold median value (female sex ratio = 0.4) considered to be</p><p>consistent with an increased probability of extinction, according to earlier population viability</p><p>analyses for the species.Wealso found a significant positive correlation between female ratio and</p><p>the proportion of young individuals in the non-breeding flocks surveyed. Our results (strongly</p><p>male-biased ASRs) support the hypothesis that the viability of Little Bustard populations in</p><p>Western Europe is threatened by an excess of female mortality, something that should be</p><p>quantified in the future, and emphasise the value of monitoring sex ratio as a population viability</p><p>indicator in species where monitoring survival is difficult to achieve.</p>