Sex Ratio and Sex-Specific Survival in Avian Populations: A Review

Abstract An analysis of sex ratio in adult birds of 196 species showed that of 308 reliable estimates, most were male-biased. Primary sex ratio (at fertilization and in clutches) was equal, but during later life it became biased due to sex-specific mortality. A surplus of males is maintained at all ages since gaining independence. Higher survival rate of males as compared with females is common, but in many populations the deviation from the equal ratio does not reach the level of significance. The most probable explanation of different bias in sex-specific mortality rate in birds and mammals (higher mortality in female birds and male mammals) is their respective sex determination systems. In both classes, the mortality rate is higher in the heterogametic sex, but in birds these are the females, and in mammals, males. Sex ratio starts to be male-biased at a very early age, soon after fledging. The higher level of natal dispersal in females in conjunction with them being heterogametic may result in their greater mortality as compared with males.