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 2 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), males are 11% larger. In both species, females forced to lay an additional egg (presumed in worse condition) overproduced daughters, whereas females receiving supplemental food before laying (presumed improved condition) overproduced sons. This sex ratio skew was larger in Mew Gull, a 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 Mew Gull, the more dimorphic species, 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.

中文翻译：

---

操纵父母条件会影响性别大小二态性不同的两种海鸥物种的育雏性别比、免疫能力和早期雏鸡死亡率

性别分配理论预测，父母应该调整他们的育雏性别比例，以最大化与父母投资相关的健康回报。性别比例的适应性调整可能是由抚养儿子和女儿的不同成本或将有限资源投资于不同性别后代的不同收益所驱动的。在这两种情况下，可能的性别比例偏差应取决于父母的状况。对于雄性大于雌性的两性异形鸟类，儿子可能不太可能长出羽毛，因为它们更容易受到食物短缺的影响，或者由于睾丸激素水平较高而导致免疫能力受损。因此，状况不佳的雌性应该过度生产女儿，以尽量减少可能的生殖失败。我们操纵了产卵的数量和产卵雌性可用的食物量，以诱导 2 种性别大小二态性不同的海鸥的状况差异。在黑头鸥（Chroicocephalus ridibundus）中，性别大小差异很小；但在 Mew Gull (Larus canus) 中，雄性要大 11%。在这两个物种中，雌性被迫多产一个卵（假定情况更糟）会过度生产女儿，而在产卵前接受补充食物的雌性（假定情况有所改善）会生产过多的儿子。这种性别比例偏斜在 Mew Gull 中更大，这是一种具有更大尺寸二态性的物种。孵化时的小鸡免疫能力与性别无关，在喂食的母亲的育雏中较高，而在最后产下的鸡蛋孵化的小鸡中较低。孵化和孵化后第 5 天之间的雏鸡存活率与其免疫能力呈正相关，但来自最后产卵的雏鸡和 Mew Gull（更多二态物种）雄性的雏鸡存活率较差。结果表明，在所研究的人群中，养育更大的性后代的成本加上父母的状况决定了育雏性别比例。适应性育雏性别比调整主要发生在产卵之前，包括根据产生羽翼未丰的小鸡的概率不同的鸡蛋性别分配。