Data from: Success despite the stress: violet-green swallows increase glucocorticoids and maintain reproductive output despite experimental increases in flight costs

Glucocorticoid steroid hormones play a central role in regulating the metabolic state of animals, especially when they cope with unanticipated stressors in their environment. The cort-adaptation hypothesis predicts that baseline concentrations of glucocorticoids are adjusted upward to match energetic needs and promote fitness when individuals are faced with physiological challenges, including those associated with reproduction. We tested the cort-adaptation hypothesis in the violet-green swallow (Tachycineta thalassina) by experimentally increasing flight costs during the offspring rearing period. Individual females were assigned to one of three treatments: light feather clipping (removal of four wing feathers), heavy feather clipping (removal of eight wing feathers) or a sham-clip control. We measured baseline corticosterone and body mass immediately prior to implementing treatments during the offspring rearing period and then 10 days after initial manipulations took place. We also quantified risk-taking behaviour, offspring feeding rate and the number of offspring fledged. Finally, we examined how treatments influenced offspring phenotype via measurements of nestling body mass and baseline corticosterone, as both measures have been associated with post-fledging survival. We found that handicapped females significantly increased baseline corticosterone between the two sampling periods, with the magnitude of change in the light clipping and heavy clipping treatments 2·5× and 6·1× greater than controls, respectively. All individuals lost mass between the two sampling periods, but the degree of loss was greater for females in both clipping treatments relative to unmanipulated controls. In contrast, we found no evidence of treatment differences in female risk-taking, offspring provisioning or in the number of offspring fledged. Offspring raised by females in both handicapped treatments did have significantly elevated baseline corticosterone relative to those in control broods, but we detected no treatment differences in offspring body mass. Our study found that handicapped females increased circulating glucocorticoids and were able to maintain critical parental care behaviours and raise a similar number of offspring as unmanipulated controls. Thus, increases in baseline cort of handicapped females appeared to have allowed them to maintain fitness despite increased physiological challenges, providing support for the cort-adaptation hypothesis.糖皮质类固醇激素（glucocorticoid steroid hormones）在调节动物代谢状态中发挥核心作用，尤其在动物应对环境中突发应激源时尤为关键。皮质酮适应假说（cort-adaptation hypothesis）预测，当个体面临包括繁殖相关在内的生理挑战时，机体会上调糖皮质激素的基础浓度，以匹配能量需求并提升个体适合度。 我们在紫绿树燕（Tachycineta thalassina）中检验了皮质酮适应假说，实验手段为在育雏期增加雌性个体的飞行成本。将受试雌性分为三组处理：轻度剪羽组（拔除4枚翼羽）、重度剪羽组（拔除8枚翼羽）以及假剪羽对照组。 我们在育雏期实施处理前即刻测定了个体的基础皮质酮（baseline corticosterone）水平与体质量，并在初始操作10天后再次开展测定。同时量化了雌性个体的冒险行为、育雏投喂率以及出飞幼鸟数量。最后，我们通过测定雏鸟体质量与基础皮质酮水平，探究了处理对幼鸟表型的影响——这两项指标均与幼鸟出飞后的存活率密切相关。 结果显示，剪羽处理的雌性个体在两次采样期间的基础皮质酮水平显著升高，轻度剪羽组与重度剪羽组的激素变化幅度分别为对照组的2.5倍与6.1倍。所有个体在两次采样间均出现体质量下降，但剪羽处理组的体质量下降幅度显著高于未受操作的对照组。 与之相对，我们未发现雌性个体的冒险行为、育雏投喂行为以及出飞幼鸟数量存在处理间显著差异。由剪羽处理雌性抚育的幼鸟，其基础皮质酮水平显著高于对照组育雏的幼鸟，但幼鸟体质量未出现处理间差异。 本研究发现，剪羽处理的雌性个体循环糖皮质激素水平升高，但仍能够维持关键的育雏行为，且抚育出的幼鸟数量与未受操作的对照组无显著差异。因此，受处理雌性的基础皮质酮升高似乎使其能够在生理挑战加剧的情况下维持适合度，为皮质酮适应假说提供了实证支持。