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.