Signal traits often evolve to communicate information about individual differences in physiological state, especially those important for fitness. As the ability to cope with stressors is an essential aspect of an animal’s fitness, it may be beneficial to advertise to conspecifics. When facing stressors, vertebrates mount a glucocorticoid stress response. However, mounting a stress response is energetically demanding and can be especially costly when breeding. Because regulation of the stress response is important to fitness, and because glucocorticoids (CORT) shares biochemical pathways with many condition-dependent ornaments, I investigated whether ventral plumage color in tree swallows—an apparent social signal—predicts CORT regulation across populations of tree swallows, Tachycineta bicolor. Specifically, I examined whether the brightness of white ventral plumage predicts three aspects of CORT regulation: baseline levels,the CORT response to a standardized stressor, and the ability to down-regulate the CORT response through negative feedback. For a population in Alaska (AK), facing harsher breeding conditions than one in New York (NY), I expected brighter ventral plumage to positively correlate with negative feedback efficacy. My results showed that in NY, birds with brighter breast feathers had significantly stronger negative feedback, but that in AK, there was no relationship between feedback efficacy and breast brightness. Neither baseline nor stress-induced CORT levels predicted color in either population. Mean negative feedback strength was weaker in NY for the favorable year of 2016 than in 2017, despite both populations facing challenging environmental conditions that year. These results suggest that bright white breast plumage in tree swallows could serve as signal trait to communicate strong negative feedback to potential mates or aggressors.