Glucocorticoids, individual quality and reproductive investment in a passerine bird

Measures of individual quality, especially energetic resources, have long been linked to investment in reproduction. However, the physiological mechanisms underlying this relationship are not well understood. In this study, we examined glucocorticoids as a potential mediator linking individual quality to investment in reproduction, because glucocorticoids have been associated with measures of both energetic resources and reproduction. We manipulated energetic resources (feather-clipping handicap) and glucocorticoids (corticosterone-soaked dermal patch) in female tree swallows, Tachycineta bicolor, and measured reproductive investment (offspring provisioning rate). Feather clipping was performed approximately 10 days before the first behavioural trial, while corticosterone patches were applied the day before the second behavioural trial. Prior to corticosterone manipulation, handicapped females provisioned their offspring at a lower rate and had lower levels of endogenous free corticosterone than control females. Also prior to the corticosterone manipulation, there was a trend for a negative association between endogenous corticosterone and offspring provisioning rate. There was no effect of the corticosterone manipulation on offspring provisioning rates, but there is some uncertainty regarding the efficacy of the manipulation. Overall, these results do not support the hypothesis that glucocorticoids are the primary physiological mechanism linking quality and reproduction, as measured by offspring provisioning behaviour. Instead, individuals facing a sustained reduction in quality lower their circulating levels of free corticosterone perhaps to mitigate the negative effects of their handicap to themselves and their offspring.