Data from: The trade-off between clutch size and egg mass in tree swallows (Tachycineta bicolor) is modulated by female body mass

Egg production is a costly component of reproduction for female birds in terms of energy expenditure and maternal investment. Because resources are typically limited, clutch size and egg mass are expected to be constrained, and this putative trade-off between offspring number and size is at the core of life history theory. Nevertheless, empirical evidence for this trade-off is equivocal at best, as individual heterogeneity in resource acquisition and allocation may hamper the detection of the negative correlation between egg number and mass within populations. Here, we investigated how female body mass and landscape composition influences clutch size, egg mass, and the relationship between these two traits. To do so, we fitted linear mixed models using data from tree swallows Tachycineta bicolor breeding in a network of 400 nestboxes located along a gradient of agricultural intensity between 2004 and 2011. Our dataset comprised 1463 broods for clutch size analyses and 4371 eggs (from 847 broods laid between 2005–2008) for egg mass analyses. Our results showed that agricultural intensity negatively impacted clutch size, but not egg mass nor the relationship between these two traits. Female mass, on the other hand, modulated the trade-off between clutch size and egg mass. For heavier females, both traits increased jointly, without evidence of a trade-off. However, for lighter females, there was a clear negative relationship between clutch size and egg mass. This work shows that accounting for individual heterogeneity in body mass allows the detection of a clutch size/egg mass trade-off that would have remained undetected otherwise. Identifying habitat and individual effects on resource allocation towards reproductive traits may help bridging the gap between predictions from theory and empirical evidence on life history trade-offs.

对于雌性鸟类而言，从能量消耗与母体投入的视角来看，产卵是繁殖过程中成本高昂的环节。由于资源通常有限，窝卵数（clutch size）与卵重（egg mass）均会受到限制，而后代数量与后代大小之间的这种假定权衡，正是生活史理论的核心议题。尽管如此，该权衡关系的实证证据至多仅能算是缺乏定论——种群内部个体在资源获取与分配上的异质性，可能会阻碍卵数与卵重之间负相关关系的检测。
本研究旨在探究雌性体重与景观组成如何影响窝卵数、卵重，以及这两个性状之间的关联。为此，我们基于2004年至2011年间沿农业集约化程度梯度布设的400个巢箱网络中繁殖的双色树燕（Tachycineta bicolor）的数据，构建了线性混合模型（linear mixed models）。本数据集包含用于窝卵数分析的1463窝样本，以及用于卵重分析的4371枚卵（采自2005至2008年间的847窝产卵样本）。
研究结果显示，农业集约化程度对窝卵数具有负向影响，但并未显著影响卵重，亦未改变这两个性状之间的关联。而雌性体重则会调控窝卵数与卵重之间的权衡关系：对于体重较大的雌性个体而言，这两个性状会协同增加，并无权衡关系的证据；但对于体重较轻的雌性个体而言，窝卵数与卵重之间则呈现出显著的负相关关系。
本研究表明，若考虑个体体重的异质性，则可检测到原本无法被发现的窝卵数与卵重之间的权衡关系。明确生境与个体效应对繁殖性状资源分配的影响，或有助于弥合生活史权衡的理论预测与实证证据之间的鸿沟。