Individual and demographic consequences of reduced body condition following repeated exposure to high temperatures

Although the lethal consequences of extreme heat are increasingly reported in the literature, the fitness costs of exposure to sublethal 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 semiarid population of White-plumed Honeyeaters, Ptilotula penicillatus, monitored between 1986 and 2012. During this 26-yr 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 d 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 semiarid 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.