Data from: Do baseline glucocorticoids simultaneously represent fitness and environmental quality in a declining aerial insectivore?

Glucocorticoids (GCs) are often interpreted as indicators of disturbance, habitat quality, and fitness in wild populations. However, since most investigations have been unable to examine habitat variability, GC levels, and fitness simultaneously, such interpretations remain largely unvalidated. We combined a quantification of two habitat types, a manipulation of foraging ability (feather-clipping just prior to nestling rearing), multiple baseline plasma GC measures, and multi-year reproductive monitoring to experimentally examine the linkages between habitat quality, GCs, and fitness in female tree swallows Tachycineta bicolor. Control females experiencing the higher early-season food resources of inland–pasture habitat laid larger clutches , but fledged an equal number but lower mass offspring compared to those in riparian–cropland habitat. Despite these differences in reproductive success, females nesting in the two habitat types did not differ in baseline GC levels at the early- or late-breeding stage. Feather-clipping reduced provisioning rate in both habitat types. However, baseline GC levels were affected in a habitat-specific way; only individuals in Inland-Pasture habitats showed an increase in GCs. Despite this difference in GC levels, the manipulation did not influence offspring mass, reproductive output, adult return rate (a proxy for survival) to the following year, or reproductive success in the subsequent year. Nonetheless, regardless of treatment, individuals with higher GC levels during the late breeding stage returned in the following year with higher GC levels at incubation, indicating a long-term effect on future GC levels. Our results indicate that environmental changes (e.g. foraging conditions) can have consequences for body condition, behaviour, and current and future baseline GC levels without concomitant influences on fitness, and that differences in fitness components between habitats may not be reflected in baseline GC levels. These results illustrate that baseline GCs may not simultaneously reflect environmental quality and fitness, potentially limiting their application in ecological and conservation settings.

糖皮质激素（Glucocorticoids, GCs）常被用作野生种群受干扰程度、栖息地质量以及适合度的指示指标。然而，由于绝大多数研究无法同时评估栖息地变异、GC水平与适合度，这类解读在很大程度上尚未得到验证。本研究结合了两种栖息地类型的量化分析、取食能力操控（在育雏前对飞羽进行修剪）、多次基础血浆GC水平检测以及多年繁殖监测，以实验方式探究雌性双色树燕（Tachycineta bicolor）的栖息地质量、GCs与适合度之间的关联。相较于河岸农田生境的个体，在内陆牧场生境（该生境早期食物资源更为丰富）的对照组雌鸟产下的窝卵数更大，但出飞后代的数量与之相当，而后代体重更低。尽管繁殖成功率存在上述差异，但两种生境中筑巢的雌鸟在繁殖早期与晚期的基础GC水平并无显著差异。飞羽修剪操作降低了两种生境下的育雏率。然而，基础GC水平的变化呈现生境特异性：仅在内陆牧场生境的个体中，GC水平出现了升高。尽管GC水平存在上述差异，但该操控并未对子代体重、繁殖产出、次年成鸟回迁率（作为生存的替代指标）以及次年繁殖成功率产生影响。尽管如此，无论接受何种处理，繁殖晚期GC水平更高的个体，在次年孵卵期的GC水平也更高，这表明GC水平存在长期的后续影响。本研究结果表明，环境变化（例如取食条件）可对身体状况、行为以及当前和未来的基础GC水平产生影响，却不会对适合度产生伴随性影响；同时，不同生境间适合度组分的差异，可能无法通过基础GC水平反映出来。本研究结果提示，基础GC水平或许无法同时反映环境质量与适合度，这可能限制了其在生态学与保护实践中的应用。