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

Adult sex ratios (ASRs) have proved to correlate with population trends, which make them

potential useful indicators of a species’ population trajectory and conservation status. We

analysed ASRs and proportion of juveniles in flocks of an endangered steppe bird, the Little

Bustard Tetrax tetrax, using surveys made during the non-breeding period in seven areas within

its Western European range (one in Portugal, four in Spain, and two in France). We found

overall male-biased ASRs, as all the seven surveyed areas showed a male-biased ASR mean value.

Five areas were below the threshold median value (female sex ratio = 0.4) considered to be

consistent with an increased probability of extinction, according to earlier population viability

analyses for the species.Wealso found a significant positive correlation between female ratio and

the proportion of young individuals in the non-breeding flocks surveyed. Our results (strongly

male-biased ASRs) support the hypothesis that the viability of Little Bustard populations in

Western Europe is threatened by an excess of female mortality, something that should be

quantified in the future, and emphasise the value of monitoring sex ratio as a population viability

indicator in species where monitoring survival is difficult to achieve.