Corticosterone predicts double-brooding in female savannah sparrows (Passerculus sandwichensis).

Given that double-brooding (rearing two broods within a season) can increase annual fecundity, it is unclear why some females in multi-brooded populations rear only one brood per season. The Quality Hypothesis proposes that double-brooded females are high quality and, thus, have sufficient energetic resources available to bear the costs of rearing two broods per season. Glucocorticoids - endocrine hormones that have a critical role in energy regulation - could reflect female quality, and, therefore, also have the potential to indicate whether a female will rear a second brood. Using 12 years of reproductive data on migratory Savannah sparrows (Passerculus sandwichensis) from a population in eastern Canada, we explored whether baseline corticosterone concentrations were correlated with measures of female quality (body condition and fat score) and whether a female's baseline corticosterone concentrations during her first brood would predict whether she attempted a second. We found weak evidence that baseline corticosterone was negatively correlated with female body condition and found strong evidence that baseline corticosterone was negatively correlated with fat score. There was weak evidence for a positive relationship between double-brooding and baseline corticosterone in females sampled during the first brood incubation stage. Additionally, there was moderate evidence to suggest that the probability of double-brooding was negatively related to baseline corticosterone in females sampled during the first brood nestling stage. Our results provide evidence that corticosterone can reflect female condition in the context of double-brooding and demonstrate the importance of considering breeding stage when assessing corticosterone concentrations in parents.