Are brood sex ratios adaptive? The effect of experimentally altered brood sex ratio on nestling growth, mortality, and recruitment

Brood sex ratios (BSRs) have often been found to be non-random in respect of parental and environmental quality, and many hypotheses suggest that non-random sex ratios can be adaptive. To specifically test the adaptive value of biased BSRs, it is crucial to disentangle the consequences of BSR and maternal effects. In multiparous species, this requires cross-fostering experiments where foster parents rear offspring originating from multiple broods, and where the interactive effect of original and manipulated BSR on fitness components are tested. To our knowledge, our study on collared flycatchers (Ficedula albicollis) is the first that meets these requirements. In this species, where BSRs had previously been shown to be related to parental characteristics, we altered the original BSR of the parents shortly after hatching by cross-fostering nestlings among trios of broods, and examined the effects on growth, mortality, and recruitment of the nestlings. We found that original and experimental BSR, as well as the interaction of the two were unrelated to the fitness components considered. Nestling growth was related only to background variables, namely brood size and hatching rank. Nestling mortality was related only to hatching asynchrony. Our results therefore do not support that the observed BSRs are adaptive in our study population. However, we cannot exclude the possibility of direct effects of experimentally altered BSRs on parental fitness, which should be evaluated in the future. In addition, studies similar to ours are required on various species to get a clearer picture of the adaptive value of non-random BSRs.

Methods
Data were collected in a nest-box breeding population of the collared flycatcher (Ficedula albicollis) in the Pilis-Visegrádi Mountains, Hungary (47°43′ N, 19°01′ E) in 2017-2021.

窝雏性比（Brood sex ratios, BSRs）常被发现与亲本质量和环境质量相关且并非随机分布，诸多假说指出非随机的性比可能具有适应性。为专门检验偏倚性窝雏性比的适应性价值，厘清窝雏性比与母体效应的影响至关重要。在多产物种中，这需要开展交叉抚育实验：由寄养亲代抚育来自多个窝巢的幼雏，并检验原始性比与操控后性比的交互效应对适合度组分的影响。据我们所知，这项针对斑姬鹟（Ficedula albicollis）的研究是首个满足上述要求的工作。此前已有研究表明，该物种的窝雏性比与亲本特征相关，我们于幼雏孵化后不久，通过在三窝巢之间交换幼雏来操控亲本的原始窝雏性比，并考察其对幼雏生长、存活与种群招募的影响。

我们发现，原始性比、实验操控后的性比，以及二者的交互作用，均与所考量的适合度组分无显著关联。幼雏生长仅与背景变量（即窝巢大小与孵化顺位）相关，幼雏存活仅与孵化异步性相关。因此，本研究种群中的观测结果并不支持窝雏性比具有适应性这一结论。不过，我们无法排除实验操控的窝雏性比对亲本适合度存在直接影响的可能性，这一点有待未来研究进一步验证。此外，针对不同物种开展类似研究，有助于更清晰地阐明非随机窝雏性比的适应性价值。

研究方法
数据采集于2017至2021年间，地点位于匈牙利皮利斯-维谢格拉德山区（47°43′ N, 19°01′ E）的斑姬鹟（Ficedula albicollis）巢箱繁殖种群。