Maternal glucocorticoids do not predict reproductive investment nor breeding success in a free-ranging bird.

The hormone corticosterone (CORT)) has been hypothesized to be linked with fitness, but the directionality of the relationship is unclear. The CORT-fitness hypothesis proposes that high levels of CORT arise from challenging environmental conditions, resulting in lower reproductive success (a negative relationship). In contrast, the CORT-adaptation hypothesis suggests that, during energetically demanding periods, CORT will mediate physiological or behavioural changes that result in increased reproductive investment and success (a positive relationship). During two breeding seasons, we experimentally manipulated circulating CORT levels in female tree swallows (Tachycineta bicolor) prior to egg laying, and measured subsequent reproductive effort, breeding success, and maternal survival. When females were recaptured during egg incubation and again during the nestling stage, the CORT levels were similar among individuals in each treatment group, and maternal treatment had no effect on indices of fitness. By considering variation among females, we found weak support for the CORT-adaptation hypothesis; there was a non-significant positive relationship between CORT levels during incubation and hatching success. During the nestling stage CORT levels were unrelated to any measure of investment or success. Within the environmental context of our study, we found little support for any relationship between maternal glucocorticoid levels and indices fitness.