Seasonal physiological responses to heat in an alpine range-restricted bird: the Cape Rockjumper (Chaetops frenatus)

Hot, dry summer conditions impose physiological stress on endotherms, yet we have a poor understanding of how endotherms seasonally adjust their costs of thermoregulation under hot conditions. We determined whether seasonal phenotypic plasticity in evaporative cooling capacity at high temperatures explained how the range-restricted Cape Rockjumper (Chaetops frenatus; hereafter 'Rockjumper'), copes with hot and dry summer temperatures of the temperate mountain peaks of southwest South Africa. We measured evaporative water loss (EWL), resting metabolic rate (RMR), and body temperature at high air temperatures (30-42 degrees C) of individuals from a wild population of Rockjumpers during winter and summer (n = 11 winter, 4 females, 7 males; n = 10 summer, 6 females, 4 males). We found Rockjumper evaporative cooling in summer imposes higher EWL (i.e. greater water costs) compared to winter, although an accompanying lack of change in RMR resulted in increased summer cooling efficiency. These patterns are similar to those observed in species that inhabit regions where summer temperatures are routinely high but the species are not water stressed. Our findings indicate that avian seasonal physiological adjustments to heat can be diverse. Further seasonal studies on thermoregulation in response to heat will greatly improve our knowledge of the functional value of traits such as evaporative cooling efficiency and heat tolerance and how they contribute to the physiological stress organisms experience in heterogenous environments.