Rising temperatures pose a grave risk to arid zone birds because they are already living close to their physiological limits and must balance water conservation against the need for evaporative cooling. We assess how extreme temperatures affect a wild population of small passerines by monitoring daily mass change in individual Jacky Winters (a small Australasian robin; Microeca fascinans) across a series of severe heatwaves that afflicted southern Australia in the summer of 2018-19. Daily maximum temperature and duration of heat exposure were negatively related to the birds’ ability to maintain body mass. At maximum temperatures ≥42^oC, birds lost 2.0% of their body mass daily and at ≥45^oC, 2.6%. Apparent mortality increased almost three-fold, and all breeding birds abandoned their nests. Nevertheless, net daily mass loss was less than might be expected from laboratory-based findings, presumably because wild Jacky Winters undertook behavioural thermoregulation. The birds also regained some mass between heatwave events and suffered no long-term reduction in body condition.