Data from: Cross-fostering eggs reveals that female collared flycatchers adjust clutch sex ratios according to parental ability to invest in offspring

Across animal taxa, reproductive success is generally more variable and more strongly dependent upon body condition for males than for females; in such cases, parents able to produce offspring in above-average condition are predicted to produce sons, whereas parents unable to produce offspring in good condition should produce daughters. We tested this hypothesis in the collared flycatcher (Ficedula albicollis) by cross-fostering eggs among nests and using the condition of foster young that parents raised to fledging as a functional measure of their ability to produce fit offspring. As predicted, females raising heavier-than-average foster fledglings with their social mate initially produced male-biased primary sex ratios, whereas those raising lighter-than-average foster fledglings produced female-biased primary sex ratios. Females also produced male-biased clutches when mated to males with large secondary sexual characters (wing patches), and tended to produce male-biased clutches earlier within breeding seasons relative to females breeding later. However, females did not adjust the sex of individuals within their clutches; sex was distributed randomly with respect to egg size, laying order, and paternity. Future research investigating the proximate mechanisms linking ecological contexts and the quality of offspring parents are able to produce with primary sex-ratio variation could provide fundamental insight into the evolution of context-dependent sex-ratio adjustment.在整个动物类群中，雄性的繁殖成功率通常比雌性更具变异性，且与身体状况的相关性更强；在此类情境下，理论预测：能够培育出高于平均质量后代的亲本应倾向于生育雄性后代，而无法培育出优质后代的亲本则应生育雌性后代。我们通过在不同鸟巢间交叉寄养鸟卵，并以亲本抚育至出飞的寄养幼鸟的身体状况，作为其培育健康后代能力的功能性衡量指标，在领姬鹟（Ficedula albicollis）中检验了这一假说。正如预测所示，与配偶共同抚育重于平均体重的寄养出飞幼鸟的雌性个体，其初始产出的初级性比（primary sex ratio）偏向雄性；而抚育轻于平均体重的寄养幼鸟的雌性个体，其初级性比则偏向雌性。雌性个体若与具有大型次级性征（secondary sexual characters，如翅斑）的雄性交配，也会产出性比偏向雄性的窝卵；且相较于繁殖较晚的雌性，繁殖季早期的雌性也倾向于产出性比偏向雄性的窝卵。然而，雌性并未调整其窝卵内单个个体的性别：后代性别与卵大小、产卵顺序以及父本身份均呈随机分布。未来若有研究探究将生态背景、亲本所能培育的后代质量，与初级性比变异联系起来的近因机制，将为情境依赖性性比调整的演化过程提供根本性的认知洞见。