Supplementary tables and analyses.Data used in the analyses. from Strategic adjustment of parental care in tree swallows: life-history trade-offs and the role of glucocorticoids

Life-history theory predicts that optimal strategies of parental investment will depend on ecological and social factors, such as current brood value and offspring need. Parental care strategies are also likely to be mediated in part by the hypothalamic–pituitary–adrenal axis and glucocorticoid hormones. Here, we present an experiment in tree swallows (<i>Tachycineta bicolor</i>), a biparental songbird with wide geographical distribution, asking whether parental care is strategically adjusted in response to signals of offspring need and brood value and if so, whether glucocorticoids are involved in these adjustments. Using an automated playback system, we carried out playbacks of nestling begging calls specifically to females in two populations differing in their brood value: a northern population in Ontario, Canada (relatively higher brood value) and a southern population in North Carolina, USA (relatively lower brood value). We quantified female offspring provisioning rates before and during playbacks and plasma corticosterone levels (cort) once during late incubation and once immediately after playbacks. Females in both populations increased feeding rates temporarily during the first 2 h of playback but the increase was not sustained for the entire duration of playback (6 h). Cort levels from samples at the end of the playback did not differ between control females and females that received playbacks. However, females that had higher increases in cort between the incubation and nestling period had greater fledging success. These results suggest that females are able to strategically respond to offspring need, although the role of glucocorticoids in this strategic adjustment remains unclear.

生活史理论（life-history theory）预测，亲本投资的最优策略将取决于生态与社会因素，例如当前窝雏价值与子代需求。亲本抚育策略亦可能部分由下丘脑-垂体-肾上腺轴（hypothalamic–pituitary–adrenal axis）与糖皮质激素（glucocorticoid hormones）介导。本研究以地理分布广泛的双亲抚育型鸣禽树燕（Tachycineta bicolor）为对象开展实验，旨在探究亲本抚育是否会根据子代需求与窝雏价值的信号进行策略性调整，以及糖皮质激素是否参与该调整过程。研究利用自动回放系统（automated playback system），针对两个窝雏价值存在差异的树燕种群的雌性个体开展雏鸟乞食叫声（nestling begging calls）回放实验：其一为位于加拿大安大略省的北方种群（窝雏价值相对更高），其二为位于美国北卡罗来纳州的南方种群（窝雏价值相对更低）。我们量化了回放实验前后以及回放过程中雌性个体的子代饲喂速率（offspring provisioning rates），并分别在孵化后期及回放实验结束后即刻，两次测定其血浆皮质酮（corticosterone，cort）水平。结果显示，两个种群的雌性个体均在回放实验开始后的前2小时内暂时提升了饲喂速率，但该提升并未在整个6小时的回放周期内持续维持。回放实验结束时采集的血浆皮质酮样本表明，对照组雌性与接受回放实验的雌性个体皮质酮水平无显著差异。然而，在孵化期至育雏期之间皮质酮水平提升幅度更高的雌性个体，其雏鸟出飞成功率（fledging success）也更高。综上结果提示，雌性树燕能够根据子代需求进行策略性响应，尽管糖皮质激素在这一策略性调整中所扮演的角色仍不明确。