Reaction norms for heat tolerance and evaporative cooling capacity do not vary across a climatic gradient in a passerine bird

There is increasing evidence for considerable phenotypic flexibility in endotherm thermal physiology, a phenomenon with far-reaching implications for the evolution of traits related to heat tolerance. Numerous studies have documented intraspecific variation in avian thermoregulatory traits, but few have revealed the shapes of thermoregulatory reaction norms or how these might vary among populations. We investigated phenotypic flexibility in the ability of a model Afrotropical passerine bird (the white-browed sparrow-weaver, Plocepasser mahali) to handle high air temperatures (T-a). We allocated birds from three sites varying by similar to 11 degrees C in mean daily summer maximum T-a to three acclimation temperature (T-accl) treatments (daytime T-accl approximate to 30 degrees C, 36 degrees C or 42 degrees C respectively; n approximate to 10 per site per T-accl). After an acclimation period of 30 days, heat tolerance and evaporative cooling capacity was quantified by exposing birds to progressively higher T-a until they approached severe hyperthermia (body temperature [T-b] = 44.5 degrees C; T-a range: 38-54 degrees C). We measured metabolic rate and evaporative water loss using open flow-through respirometry, and T-b using temperature-sensitive passive-integrated transponder tags. Hyperthermia threshold T-a (T-a,T-HT) was significantly higher and T-b significantly lower in birds acclimated to the hottest T-accl compared to those from milder acclimation treatments. Population (i.e., site of capture) was not a significant predictor of any thermoregulatory variables or hyperthermia threshold T-a(T-a,(HT)) after acclimation, revealing that the shape of reaction norms for heat tolerance and evaporative cooling capacity does not vary among these three populations.