Physiological changes in response to environmental stressors can reveal cryptic effects of disturbance that can potentially lead to species decline. However, such responses may vary with life history. We used heart rate telemetry to continuously and instantaneously measure energy expenditure in response to human-mediated disturbance in a free-living breeding population of the endangered [International Union for Conservation of Nature, vulnerable] black-capped vireo Vireo atricapilla (n=10). Heart rate predicted 84% of energy expenditure as determined by respirometry (n=3). Each bird mounted with a 0.5 g heart rate transmitter were subjected to standardized disturbance trials for 1 or 4 h during the day, and for 1 h at night. Only 1-h daytime disturbances elicited an increase in heart rate but this was not significant when compared with control no-disturbance periods. Our findings suggest that black-capped vireos quickly acclimate to a limited amount of human disturbance during the breeding season, which may be an adaptive response for any ‘fast-living’ species with a short life span and a short and synchronized breeding season. Because similar results were reported for another fast-living species, the common white-eyed vireo Vireo griseus, we suggest that life-history traits are stronger predictors of short-term physiological responses to disturbances than population status.