Data from: Egg size and offspring performance in the collared flycatcher (Ficedula albicollis): a within-clutch approach

Adaptive within-clutch allocation of resources by laying females is an important focus of evolutionary studies. However, the critical assumption of these studies, namely that within-clutch egg-size deviations affect offspring performance, has been properly tested only rarely. In this study, we investigated effects of within-clutch deviations in egg size on nestling survival, weight, fledgling condition, structural size and offspring recruitment to the breeding population in the collared flycatcher (Ficedula albicollis). Besides egg-size effects, we also followed effects of hatching asynchrony, laying sequence, offspring sex and paternity. There was no influence of egg size on nestling survival, tarsus length, condition or recruitment. Initially significant effect on nestling mass disappeared as nestlings approached fledging. Thus, there seems to be limited potential for a laying female to exploit within-clutch egg-size variation adaptively in the collared flycatcher, which agrees with the majority of earlier studies on other bird species. Instead, we suggest that within-clutch egg-size variation originates from the effects of proximate constraints on laying females. If true, adaptive explanations for within-clutch patterns in egg size should be invoked with caution.

产卵雌性对窝内资源的适应性分配，是进化生物学研究的重要方向之一。然而，这类研究的核心假设——即窝内卵大小偏差会影响子代表现——却极少得到严格检验。本研究以领捕蝇鶲（Ficedula albicollis）为研究对象，探讨了窝内卵大小偏差对雏鸟存活率、体重、离巢身体状况、躯体大小以及子代招募进入繁殖种群的影响。除卵大小的影响外，本研究还分析了孵化异步性、产卵顺序、子代性别以及父权对上述指标的作用。研究结果显示，卵大小对雏鸟存活率、跗跖长度、身体状况以及繁殖招募均无显著影响。卵大小对雏鸟体重的初始显著效应，会随着雏鸟临近离巢期而消失。因此，在领捕蝇鶲中，产卵雌性利用窝内卵大小变异进行适应性分配的潜力似乎十分有限，这一结论与此前针对其他鸟类的多数研究结果一致。取而代之的是，我们提出窝内卵大小变异实则源于产卵雌性所面临的近端限制因素（proximate constraints）的影响。若该结论成立，针对窝内卵大小格局的适应性解释应当谨慎援引。