Data from: Experimental brood enlargement differentially influences the magnitude of the corticosterone stress response in closely related, co‐occurring songbirds

1. Rearing environments can shape offspring phenotype across taxa, yet little is known about how brood size influences hypothalamic-pituitary axis functioning, whether its expression trades off with growth, and the degree to which these relationships vary between species. 2. We evaluated how brood size influenced nestling physiological (glucocorticoids) and somatic traits (growth), and the extent to which these relationships differed between two closely related, sympatric songbirds when experiencing identical rearing environments. Specifically, we used a cross-fostering approach to alter brood size and create an experimental gradient of nestmate competition, and then tested whether experimentally manipulated brood sizes resulted in nestlings with altered concentrations of corticosterone and if corticosterone responses traded off with growth. 3. Nestlings of both species experienced elevated concentrations of baseline and stressor-induced corticosterone when raised in enlarged broods, relative to control and reduced broods, but neither measurement was found to trade off with growth or be linked to survival to fledging. 4. In contrast, we found divergence in the magnitude of the corticosterone stress response between species across all brood treatments, with greater stressor-induced corticosterone concentrations found in the Violet-green Swallow (Tachycineta thalassina) relative to the closely related Tree Swallow (T. bicolor). 5. Our study demonstrated that brood size can lead to changes in offspring corticosterone concentrations in swallows, and that nestlings of sympatric species, even those that are closely related and ecologically similar, can diverge in their corticosterone stress response when experiencing identical rearing conditions. 6. We conclude that corticosterone appears to play a key role for balancing energetic demands that arise in the face of nestmate competition in Tachycineta swallows, and that elevated concentrations of corticosterone may enhance offspring survival during challenging environmental conditions, such as when brood competition is strong.

1. 跨各类类群而言，抚育环境均可塑造后代的表型，但目前学界对窝雏数 (brood size) 如何影响下丘脑-垂体轴 (hypothalamic-pituitary axis) 功能、该轴的表达是否与生长存在权衡，以及这些关联在不同物种间的变异程度仍知之甚少。
2. 本研究旨在评估窝雏数对雏鸟生理性状（糖皮质激素，glucocorticoids）与躯体性状（生长，growth）的影响，以及在相同抚育环境下，两种亲缘关系相近的同域鸣禽之间的上述关联存在多大差异。具体而言，我们采用交叉抚育 (cross-fostering) 手段调整窝雏数，构建了巢内同伴竞争的实验梯度，随后检验经实验操控的窝雏数是否会导致雏鸟的皮质酮 (corticosterone) 浓度发生改变，以及皮质酮响应是否与生长存在权衡。
3. 相较于对照组与窝雏数减少的实验组，两种雏鸟在窝雏数增加的环境中饲养时，其基础皮质酮与应激诱导皮质酮浓度均有所升高，但未发现这两项指标与生长存在权衡，亦未与离巢前的存活率存在关联。
4. 与之相反，在所有窝雏数处理组中，两种间的皮质酮应激响应强度均存在分化：相较于亲缘关系相近的树燕 (Tree Swallow, T. bicolor)，紫绿燕 (Violet-green Swallow, Tachycineta thalassina) 的应激诱导皮质酮浓度更高。
5. 本研究表明，燕类的窝雏数可导致后代皮质酮浓度发生改变；且即便同域分布的物种亲缘关系相近、生态位相似，其雏鸟的皮质酮应激响应在相同抚育条件下仍可出现分化。
6. 我们由此得出结论：皮质酮在燕属 (Tachycineta) 鸟类应对巢内同伴竞争所产生的能量需求平衡中发挥关键作用，且在巢内竞争激烈等挑战性环境条件下，升高的皮质酮浓度或可提升后代的存活率。