Although the lethal consequences of extreme heat are increasingly reported in the literature, the fitness costs of exposure to sub-lethal high air temperatures, typically identified in the 30- 40°C range, are poorly understood. We examine the effect of high (≥35°C) daily maxima on body condition of a semi-arid population of white-plumed honeyeaters Ptilotula penicillatus monitored between 1986 and 2012. During this 26 year period temperature has risen, on average, by 0.06°C each year at the site, the frequency of days with thermal maxima ≥35°C has increased and rainfall has declined. Exposure to high temperatures affected body condition of white-plumed honeyeaters, but only in low rainfall conditions. There was no effect of a single day of exposure to temperatures ≥35°C but repeated exposure was associated with reduced body condition: 3.0% reduction in body mass per day of exposure. Rainfall in the previous 30 days ameliorated these effects, with reduced condition evident only in dry conditions. Heat-exposed males with reduced body condition were less likely to be recaptured at the start of the following spring; they presumably died. Heat-exposed females, regardless of body condition, showed lower survival than exposed males, possibly due to their smaller body mass. The higher mortality of females and smaller males exposed to temperatures ≥35°C may have contributed to the increase in mean body size of this population over 23 years. Annual survival declined across time concomitant with increasing frequency of days ≥35°C and decreasing rainfall. Our study is one of few to identify a proximate cause of climate change related mortality, and associated long-term demographic consequence. Our results have broad implications for avian communities living in arid and semi-arid regions of Australia, and other mid-latitudes regions where daily maximum temperatures already approach physiological limits in regions affected by both decreased precipitation and warming.