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...性分配理论（Sex allocation theory）预测，亲本应依据亲本投资（parental investment）调整其窝卵性比（brood sex ratio），以实现适合度收益（fitness returns）的最大化。性比的适应性调节（adaptive adjustment of sex ratio）可能源于抚育雌雄后代的差异化成本（differential costs），或是将有限资源（limited resources）投入不同性别后代所带来的差异化收益。在这两种情形下，潜在的性比偏倚均应取决于亲本的身体状态（parental condition）。对于雄性体型大于雌性的性二型鸟类（sexually dimorphic birds）而言，雄性后代更难成功离巢（fledge）——这可能是因为它们更易受食物短缺影响，或是由于睾酮（testosterone）水平较高而导致免疫能力（immunocompetence）受损。因此，身体状况较差的雌鸟应多产雌性后代，以尽可能降低繁殖失败（reproductive failure）的风险。我们通过调控两种存在性体型二态性（sexual size-dimorphism）差异的鸥类的产卵数量，以及产卵雌鸟可获得的食物量，来诱导其身体状态产生差异。在红嘴鸥（Black-headed Gull，Chroicocephalus ridibundus）中，体型性别差异……