Desert birds routinely experience air temperatures that exceed their normal body temperatures, and under conditions of intense solar radiation may need to defend body temperatures 15-20 °C below operative temperature. Relatively little is known about the upper limits to avian heat tolerance and evaporative cooling capacity. As part of a collaborative project with Blair Wolf (University of New Mexico), we have recently examined the interactions between body temperature, evaporative water loss and metabolic rate at high air temperatures in arid-zone bird communities in the Kalahari Desert. This study is providing novel insights into how evaporative cooling capacity and heat tolerance vary with body mass, and factors such as the importance of respiratory versus cutaneous evaporative heat loss.

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We have also investigated intraspecific variation in heat tolerance among populations of White-browed Sparrow-weavers across a 10 °C gradient in maximum summer air temperatures. Sparrow-weavers at a hot desert site showed significantly greater heat tolerance in summer compared to populations at cooler sites, but this difference was not evident in winter. These data provide the first evidence for seasonal acclimatization in avian heat tolerance and evaporative cooling capacity, and raise a number of questions regarding the roles of phenotypic plasticity versus local adaptation as determinants of physiological variation.  A better understanding of the processes affecting birds’ physiological capacities to cope with extreme heat is critical for predicting their responses to higher maximum temperatures and more frequent heat waves, and testing the assumption implicit in climate-envelope models that birds will not be able to persist in future climates hotter than those they presently occupy.