Seasonal and geographical variation in heat tolerance and evaporative cooling capacity in a passerine bird

Intraspecific variation in avian thermoregulatory responses to heat stress has received little attention, despite increasing evidence that endothermic animals show considerable physiological variation among populations. We investigated seasonal (summer versus winter) variation in heat tolerance and evaporative cooling in an Afrotropical ploceid passerine, the white-browed sparrow-weaver (Plocepasser mahali; similar to 47 g) at three sites along a climatic gradient with more than 10 degrees C variation in mid-summer maximum air temperature (T-a). We measured resting metabolic rate (RMR) and total evaporative water loss (TEWL) using open flow-through respirometry, and core body temperature (T-b) using passive integrated transponder tags. Sparrow-weavers were exposed to a ramped profile of progressively higher T-a between 30 and 52 degrees C to elicit maximum evaporative cooling capacity (N=10 per site per season); the maximum T-a birds tolerated before the onset of severe hyperthermia (T-b approximate to 44 degrees C) was considered to be their hyperthermia threshold T-a (T-a, HT). Our data reveal significant seasonal acclimatisation of heat tolerance, with a desert population of sparrow-weavers reaching significantly higher T-a in summer (49.5 +/- 1.4 degrees C, i.e. higher T-a, HT) than in winter (46.8 +/- 0.9 degrees C), reflecting enhanced evaporative cooling during summer. Moreover, desert sparrow-weavers had significantly higher heat tolerance and evaporative cooling capacity during summer compared with populations from more mesic sites (T-a, HT=47.3 +/- 1.5 and 47.6 +/- 1.3 degrees C). A better understanding of the contributions of local adaptation versus phenotypic plasticity to intraspecific variation in avian heat tolerance and evaporative cooling capacity is needed for modelling species' responses to changing climates.