In birds, possible explanations for a bias in brood sex ratio include differential cost of rearing nestlings of different sexes, and different parental fitness returns related to offspring sex. We studied brood sex ratio of Magellanic penguins, Spheniscus magellanicus, in Puerto Deseado for 3 years. Our objectives were to compare the growth curves and energetic costs of rearing nestlings of different sexes, and to evaluate the possible implications of environmental and parental condition in the establishment of a bias in the brood sex ratio. We also investigated the relationship between hatching order and sex, and its impact on brood survival. Asymptotic mass was 11.41% higher for males than for females. The energetic cost of feeding male nestlings was slightly higher than for feeding females, but the difference in energy requirements was only 2.6% of the total energy budget. During the 3 years, brood sex ratio was 0.53, and almost constant within years over the raising period, showing no sex allocation during feeding. Sea surface temperature, which is linked to higher prey abundance when colder, explained brood sex ratio at fledging. The sex ratio was male-biased during the coldest year. Parental body condition was not an important variable explaining brood sex ratio. There was no bias in nestling sex with respect to hatching order and nestling survival was not related to nestling sex. We conclude that, even though the cost of feeding male offspring is higher, it only involves a small fraction of total cost of raising nestlings and might not be responsible for an adaptive bias in the sex ratio of nestlings for this species. However, during good oceanic conditions, females might bias their brood sex ratio towards males, thereby potentially gaining an advantage by raising good-quality males.

在鸟类中，对育雏性别比例偏差的可能解释包括饲养不同性别雏鸟的不同成本，以及与后代性别相关的不同亲本健康回报。我们研究了麦哲伦企鹅Spheniscus magellanicus 的育雏性别比例, 在波多黎各 Deseado 工作了 3 年。我们的目标是比较不同性别雏鸟的生长曲线和能量消耗，并评估环境和亲本条件对建立育雏性别比偏差的可能影响。我们还调查了孵化顺序和性别之间的关系，及其对育雏存活率的影响。男性的渐近质量比女性高 11.41%。喂养雄性雏鸟的能量成本略高于喂养雌性雏鸟，但能量需求的差异仅为总能量预算的 2.6%。3年间，育雏性别比为0.53，在饲养期间几乎保持不变，在饲养期间没有性别分配。海面温度，这与较冷时较高的猎物丰度有关，解释了雏鸟的育雏性别比。在最冷的年份，性别比例是男性偏向的。父母的身体状况不是解释育雏性别比的重要变量。雏鸟性别在孵化顺序方面没有偏见，雏鸟存活率与雏鸟性别无关。我们得出的结论是，即使喂养雄性后代的成本更高，它只涉及饲养雏鸟总成本的一小部分，并且可能不是该物种雏鸟性别比例的适应性偏差的原因。然而，在良好的海洋条件下，雌性可能会偏向雄性，从而通过饲养优质雄性而获得优势。父母的身体状况不是解释育雏性别比的重要变量。雏鸟性别在孵化顺序方面没有偏见，雏鸟存活率与雏鸟性别无关。我们得出的结论是，即使喂养雄性后代的成本更高，它只涉及饲养雏鸟总成本的一小部分，并且可能不是该物种雏鸟性别比例的适应性偏差的原因。然而，在良好的海洋条件下，雌性可能会偏向雄性，从而通过饲养优质雄性而获得优势。父母的身体状况不是解释育雏性别比的重要变量。雏鸟性别在孵化顺序方面没有偏见，雏鸟存活率与雏鸟性别无关。我们得出的结论是，即使喂养雄性后代的成本更高，它只涉及饲养雏鸟总成本的一小部分，并且可能不是该物种雏鸟性别比例的适应性偏差的原因。然而，在良好的海洋条件下，雌性可能会偏向雄性，从而通过饲养优质雄性而获得优势。它只涉及饲养雏鸟总成本的一小部分，可能不会导致该物种雏鸟性别比例的适应性偏差。然而，在良好的海洋条件下，雌性可能会偏向雄性，从而通过饲养优质雄性而获得优势。它只涉及饲养雏鸟总成本的一小部分，并且可能不会导致该物种雏鸟性别比例的适应性偏差。然而，在良好的海洋条件下，雌性可能会偏向雄性，从而通过饲养优质雄性而获得优势。