Avian thermoregulation in the heat: scaling of heat tolerance and evaporative cooling capacity in three southern African arid-zone passerines

Many birds can defend body temperature (T-b) far below air temperature (T-a) during acute heat exposure, but relatively little is known about how avian heat tolerance and evaporative cooling capacity varies with body mass (M-b), phylogeny or ecological factors. We determined maximum rates of evaporative heat dissipation and thermal end points (T-b and T-a associated with thermoregulatory failure) in three southern African ploceid passerines, the scaly-feathered weaver (Sporopipes squamifrons, M-b approximate to 10 g), sociable weaver (Philetairus socius, M-b approximate to 25 g) and white-browed sparrow-weaver (Plocepasser mahali, M-b approximate to 40 g). Birds were exposed to a ramped profile of progressively increasing T-a, with continuous monitoring of behaviour and T-b used to identify the onset of severe hyperthermia. The maximum T-a birds tolerated ranged from 48 degrees C to 54 degrees C, and was positively related to M-b. Values of T-b associated with severe heat stress were in the range of 44 to 45 degrees C. Rates of evaporative water loss (EWL) increased rapidly when T-a exceeded T-b, and maximum evaporative heat dissipation was equivalent to 141-222% of metabolic heat production. Fractional increases in EWL between T-a < 40 degrees C and the highest Ta reached by each species were 10.8 (S. squamifrons), 18.4 (P. socius) and 16.0 (P. mahali). Resting metabolic rates increased more gradually with T-a than expected, probably reflecting the very low chamber humidity values we maintained. Our data suggest that, within a taxon, larger species can tolerate higher T-a during acute heat stress.