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 ar...