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

1. 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. 2. 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. 3. 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. 4. 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 2.59 and 6.19 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. 5. 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. 6. 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.