Abstract
In a number of bird species, the sex ratio of the broods is not random, instead it is related to parental quality and environmental conditions. Several hypotheses have been proposed to explain this phenomenon, the so called sex ratio adjustment. According to these hypotheses, sex ratio adjustment is expected to evolve when the fitness benefit an offspring confers to the parents changes with ecological or social factors in a sex-specific way. Though many correlative and experimental studies support these hypotheses, there are still unresolved problems. In our paper, we provide details on the hypotheses related to sex ratio adjustment and explanations for the differences observed in sex ratio patterns between populations and years. Finally, we discuss the importance of sex ratio adjustment for species conservation.