Data from: Troubleshooting the potential pitfalls of cross-fostering

1. Cross-fostering is the transfer of offspring between their natal environment and a new social environment. This method allows researchers to disentangle the genetic and interacting environmental effects that influence phenotypes, and is popular in both wild and laboratory studies. Here, we discuss three factors that might bias cross-fostering and influence ecological and evolutionary conclusions if not accommodated. 2. First, cross-fostering tends to be spatially and temporally non-random because heterogeneous breeding conditions can result in clustered breeding attempts. Second, cross-fostering will often change the brood composition because the exchanged broods are unlikely to be precisely matched in age, size, and composition. Third, some methods can introduce bias by using a systematically structured subset of the population, leading to a systematically structured data-set. 3. We use a 12-year case study of wild house sparrows (Passer domesticus) to demonstrate how to identify these biases with statistical modelling and how to adjust the cross-fostering protocol according to the identified biases. 4. In our dataset, cross-fostered nestlings were more likely to survive than non-cross-fostered nestlings, but post-fledging and overall survival were not affected. Survival differed between cross-fostering treatments, partially due to temporally non-random breeding conditions and non-random offspring selection, demonstrating two of the three forms of bias in data from a wild population. 5. In all cases, we suggest using statistical models to examine whether cross-fostering opportunities and offspring fitness are affected by non-random breeding, changes to the brood composition, and biased methodology. We provide guidelines for optimising a cross-fostering design and reducing inherent bias.

1. 交叉抚育（cross-fostering）指将幼崽从其出生环境转移至新的社会环境中的实验操作。该方法可帮助研究者厘清影响表型的遗传效应与互作环境效应，在野生种群与实验室研究中均应用广泛。本文将探讨三类若未加以控制便会干扰交叉抚育实验、进而影响生态学与进化生物学结论的偏倚因素。
2. 第一，交叉抚育往往存在空间与时间维度上的非随机性，因为异质性的繁殖条件可能导致繁殖尝试呈现集群分布特征；第二，交叉抚育通常会改变窝雏群组成，因为交换的幼崽群在年龄、体型与个体组成上难以实现精准匹配；第三，部分研究方法会通过系统性选取种群子集引入偏倚，最终得到结构化的数据集（dataset）。
3. 我们以一项为期12年的野生家雀（Passer domesticus）野外研究案例为例，展示如何通过统计建模识别上述偏倚，以及如何根据识别出的偏倚优化交叉抚育实验方案。
4. 在本数据集中，经交叉抚育的雏鸟存活率显著高于未接受交叉抚育的雏鸟，但出飞后存活率与总存活率并未受到显著影响。不同交叉抚育处理组间的存活率存在差异，这部分源于繁殖条件的时间非随机性与后代选择的非随机性，验证了野生种群观测数据中三类偏倚中的两类。
5. 综上，我们建议通过统计模型检验非随机繁殖、窝雏群组成改变与有偏实验方法是否会对交叉抚育实施机会与后代适合度产生影响。同时我们提供了优化交叉抚育实验设计、降低固有偏倚的指南。