1. It has been proposed that life histories have coevolved with a suite of physiological and behavioural adaptations, termed pace-of-life syndromes (POLS). Here, we hypothesise that basal concentration of blood glucose (G0), a major source of energy circulating in vertebrate blood, may constitute a key component of POLS. 2. To test this hypothesis, we measured G0 in 30 passerine species and tested its covariation with body mass and other life history traits. Importantly, body mass is a major life history determinant and, when its effect is controlled for, there may be no single fast-slow life history continuum in birds comprising both fecundity and lifespan. Hence, we used individual life history traits, rather than principal component analysis, to characterise life history variation in our analysis. 3. In support of G0 life history coevolution, we found G0 to be negatively correlated with body mass and positively with reproductive investment in a single clutch across 30 passerine species. Higher G0 in females suggests that the energy demands of clutch production and incubation may be an important selection force driving coevolution of G0 with reproductive output. 4. In contrast, G0 was not associated with maximum lifespan, suggesting that high G0 may not constrain evolution of longevity. This implies that long-lived species can evolve physiological adaptations preventing harmful effects of high glucose concentrations, known to cause pathologies and accelerate ageing. 5. In addition, G0, but not basal metabolic rate (BMR), was negatively correlated with migration distance, attesting to evolutionary changes in energy metabolism in long distance migrants. Our results further suggest that the links between body mass, reproduction and G0 are not mediated by BMR and that G0 is associated with fast-slow life history variation more closely than available BMR data. 6. A species life history is determined to a great extent by body mass. When this effect is controlled for, only those traits related to reproduction (but not lifespan) constitute the principal axis of life history variation in birds. Hence, the coevolution of G0 with body mass and reproductive output evidenced in our study indicates that G0 constitutes an important physiological component of POLS.