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 (Ta). We allocated birds from three sites varying by ~ 11 °C in mean daily summer maximum Ta to three acclimation temperature (Taccl) treatments (daytime Taccl ≈ 30 °C, 36 °C or 42 °C respectively; n ≈ 10 per site per Taccl). After an acclimation period of 30 days, heat tolerance and evaporative cooling capacity was quantified by exposing birds to progressively higher Ta until they approached severe hyperthermia (body temperature [Tb] = 44.5 °C; Ta range: 38-54 °C). We measured metabolic rate and evaporative water loss using open flow-through respirometry, and Tb using temperature-sensitive passive-integrated transponder tags. Hyperthermia threshold Ta (Ta,HT) was significantly higher and Tb significantly lower in birds acclimated to the hottest Taccl 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 Ta (Ta,HT) after acclimation, revealing that the shape of reaction norms for heat tolerance and evaporative cooling capacity does not vary among these three populations.