Metabolic rate (MR) quantifies the ‘rate of living’ of aerobic organisms and represents a fundamental physiological measure used in many ecological and evolutionary studies. MR is typically measured in intact organisms confined in measurement chambers for long periods of time (respirometry). It is a well-established technique and can record real-time O₂ consumption. However, prolonged confinement may be stressful for individuals, increasing stress hormone concentrations and certain behaviors, thereby biasing MR measurements. Recently a technique to measure MR in blood cells became available, opening the possibility to assess MR in free-living and/or vulnerable individuals while minimizing exposure to stressful conditions. Yet it has remained unclear whether MR recorded in blood cells yields comparable information as traditional respirometry. We took both measures in captive great tits (Parus major) by collecting a blood sample before and after 2.5 hrs of respirometry during daytime. The two blood samples allowed us to record red blood cell MR and plasma corticosterone concentrations, while respirometry and video recordings monitored locomotor activity and whole-organism MR. Many individuals had high, stress-induced concentrations of corticosterone. Corticosterone concentrations were positively associated with locomotor activity, suggesting that both parameters reflect individual stress levels. Importantly, whole-organism and red blood cell MR were positively correlated only in individuals with low corticosterone concentrations. Our results indicate that red blood cell MR is a suitable alternative to respirometry if subjects are not stressed. They also highlight the importance of accounting for physiological and behavioral responses of individuals during respirometry.