It has been suggested that sexual size dimorphism (SSD) may influence sex ratios at different life stages. Higher energy requirements during growth associated with larger body size could lead to a greater mortality of the larger sex and ultimately to an overproduction of the smaller sex. To explore the associations between SSD and hatching and fledging sex ratio we performed a species‐level analysis and a phylogenetically controlled analysis, based on 83 bird species. Overall, there was a significant inverse relationship between the degree of SSD and the proportion of males at hatching and fledging. Sex‐specific mortality related to SSD showed a weak but persistent negative tendency, suggesting a mortality bias towards the larger sex. These results suggest that changes in relation to SSD may take place mainly at the conception stage, but could be adjusted during growth. However, conclusions should be treated cautiously as these relationships weaken when additional variables are considered.

This chapter uses recent experimental and observational studies of birds to explore patterns of sex-specific offspring vulnerability (increased mortality and reduced fledging mass under poor conditions) in relation to sexual size dimorphism (SSD). The results show size-dependent modulation of male fledgling mass but size-independent mass reduction in females. Overall, growth is more phenotypically plastic in males than in females. Comparisons of fledging mass reached in ‘good’ and ‘poor’ environments suggest that having to grow large is mainly disadvantageous when coupled with the male phenotype. Differences in environmental sensitivity between the two sexes during ontogeny, either in the form of increased mortality or reduced body size, will tend to reduce dimorphism during development, affecting adult SSD. These results suggest that environmental conditions during ontogeny contribute significantly to variation in SSD within bird species, particularly when comparisons are made among environments or between generations.

Adaptive sex allocation has frequently been studied in sexually size dimorphic species, but far less is known about patterns of sex allocation in species without pronounced sexual size dimorphism. Parental optimal investment can be predicted under circumstances in which sons and daughters differ in costs and/or fitness returns. In common terns Sterna hirundo, previous studies suggest that sons are the more costly sex to produce and rear. We investigated whether hatching and fledging sex ratio and sex‐specific chick mortality correlated with the ecological environment (laying date, clutch size, hatching order and year quality) and parental traits (condition, arrival date, experience and breeding success), over seven consecutive years. Population‐wide sex ratios and sex‐specific mortality did not differ from parity, but clutch size, mass of the father, maternal breeding experience and to some extent year quality correlated with hatching sex ratio. The proportion of sons tended to increase in productive years and when the father was heavier, suggesting the possibility that females invest more in sons when the environmental and the partner conditions are good. The proportion of daughters increased with clutch size and maternal breeding experience, suggesting a decline in breeding performance or a resources balance solved by producing more of the cheaper sex. No clear patterns of sex‐specific mortality were found, neither global nor related to parental traits. Our results suggest lines for future studies on adaptive sex allocation in sexually nearly monomorphic species, where adjustment of sex ratio related to parental factors and differential allocation between the offspring may also occur.