Brood sex ratio, early chick survival, and cell-mediated immunity measurements for 3 experimental groups of Larus canus and Chroicocephalus ridibundus pairs

Sex allocation theory predicts that parents should adjust their brood sex
ratio to maximize fitness returns in relation to parental investment.
Adaptive adjustment of sex ratio may be driven by differential costs of
rearing sons and daughters or differential benefits of investing limited
resources into offspring of different sex. In both cases, possible sex
ratio bias should depend on parental condition. For sexually dimorphic
birds with males larger than females, sons may be less likely to fledge
since they are more vulnerable to food shortages or because they have
impaired immunocompetence due to higher testosterone levels. Poor
condition females should thus overproduce daughters to minimize possible
reproductive failure. We manipulated the number of eggs laid and the
amount of food available to laying females to induce differences in the
condition in two gull species differing in sexual size-dimorphism. In the
Black-headed Gull (Chroicocephalus ridibundus), sexual size differences
are marginal, but in the Mew Gull (Larus canus, MG) males are 11% larger.
In both species, females forced to lay an additional egg (presumed in
worse condition) overproduced daughters, while females receiving
supplemental food before laying (presumed improved condition) overproduced
sons. This sex ratio skew was larger in MG, species with larger size
dimorphism. Chick immunocompetence at hatching was unrelated to sex, being
higher in broods of fed mothers and lower for chicks hatched from
last-laid eggs. Chick survival between hatching and day 5 post-hatch was
positively related to their immunocompetence, but chicks from last-laid
eggs and males of the more dimorphic species (MG) survived less well.
Results indicate that costs of raising larger sex offspring coupled with
parental condition shape brood sex ratio in populations studied. Adaptive
brood sex ratio adjustment occurs mostly before egg-laying and includes
differential sex allocation in eggs depending on the probability of
producing a fledged chick.