A paper came out last month from Sarah Pryke and Lee Rollins (link to full text here) investigating the evolutionary theory predictions of mothers adjusting sex-ratios. The theory in short would suggest that mothers adjust the ratio when the gain of fitness or costs of rearing differs between the sexes. The goal is to maximize fitness. If sons for example are more costly to rear and the female is in a low quality environment with poor body condition she should produce fewer sons. On the other hand, theory predicts that when resources are high the sex-ratio should favor the sex that will reap the highest benefits. This is just a theory and there are many studies in a variety of vertebrates with variations in sex-ratios in response to conditions.
This particular paper pertains to the blue-faced parrotfinch (Erythrura trichroa) which is not sexually dimorphic. This study is particularly interesting because it separates two variables possibly affecting sex-ratio adjustment: maternal condition and rearing environment (nutritional quality). Adult blue-faced parrotfinches are pretty resistant to changes in diet. This means that the treatment of low or high quality diet will change the rearing environment but not the parental body condition.
The authors found that the sex-ratio resulting from low quality diets was around 73% male while the high quality diets resulted in 48% male. Beyond that, female offspring on low quality diets had a much great incidence of mortality when compared with males. These two pieces of evidence taken together suggest that moms adjust to have more sons because the males will fare better in the low quality environment.
So…..why do males fare better in low quality environments? There is no significant difference in birth weight and again the blue-faced parrotfinch is not sexually dimorphic. There should be no difference in the cost to the mother from having males or females. The paper brings up the point of the heterogametic sex being the weaker sex because of sex-linked recessive alleles. (female birds are ZW) This phenomenon can be seen in other monomorphic birds as well.