Annual egg productivity predicts female-biased mortality in avian species

Among avian species, the differential cost entailed by either sex in
competition for mates have been regarded as the main evolutionary
influence on sex differences in mortality rates. However, empirical
evidence suggests that sex-biased adult mortality is mainly related to
differential energy investment in gamete production, a greater annual mass
devoted to egg production leading to higher female mortality. We
explicitly tested the generality of this pattern in a comparative
framework. Annual egg production can be relatively large in some species
(up to 200% of female body mass) and annual mortality is generally biased
towards females. We showed that greater annual egg productivity resulted
in higher mortality rates of females relative to males. Mating system was
secondarily important, species where males were more involved in mating
competition having more equal mortality rates between the sexes. However,
both traits explained only a limited fraction of the interspecific
variation in female-biased mortality. Other traits, such as sexual size
dimorphism and parental care, had much weaker influences on female-biased
mortality. Our results suggest that both annual mass devoted to gamete
production by females and mating system contribute to the evolution of the
fundamental life-history trade-off between reproduction and survival in
avian taxa.