The roles of phylogeny and body size in avian heat stress physiology, and how they interact to set the upper limits to heat dissipation capacity, are largely unexplored. Determining thermal end points and maximum capacity for evaporative heat dissipation in species from diverse ecological guilds and evolutionary clades is vital for understanding species-specific vulnerability to future climatic scenarios. I measured evaporative water loss (EWL), resting metabolic rate (RMR) and body temperature (Tb) in three arid-zone passerines and three columbids of varying mass, namely the scaly-feathered weaver (Sporopipes squamifrons, ~11 g, SFW), sociable weaver (Philetairus socius, ~26 g, SW), white-browed sparrow weaver (Plocepasser mahali, ~40 g, WBW), Namaqua dove (Oena capensis, ~37 g, ND), laughing dove (Spilopelia senegalensis, ~89 g, LD) and Cape turtle dove (Streptopelia capicola, ~148 g, CTD) at maximum air temperatures (Ta) of 48–60°C. I found that evaporative water loss increased approximately linearly in all six species above a Ta of ~ 40 °C, which resulted in SFW, SW, WBW, ND, LD and CTD dissipating a maximum of 140, 220, 190, 498, 218 and 231 % of metabolic heat loads at the highest Tas respectively. All six species used facultative hyperthermia at high Tas and were able to regulate Tb up to and just beyond Tb = 45 °C. At the highest Tas experienced, passerines exhibited uncontrolled increases in Tb above 45 °C, resulting in 57, 100 and 100 % of SFW, SW and WBW respectively, reaching thermal limits at Ta = 48, 52 and 54 °C. Very few doves exhibited uncontrolled hyperthermia or reached thermal limits at their highest respective test Tas (Ta = 56, 68 and 60 °C in CTD, LD and ND respectively), suggesting that these birds could potentially survive higher Tas, and that lethal Tb was marginally higher than my conservative estimations. A conventional analysis found significant differences between doves and passerines in the slopes of EWL as well as the magnitude of the change in RMR, EWL and Tb between Ta = 35 and 48 °C. However, once phylogeny was controlled for, these differences were shown to be a result of phylogenetic inertia. Both a conventional analysis and a phylogenetic independent contrast (PIC) found a significant effect of body mass on slope of EWL, change in EWL (PIC only) and change in Tb between Ta = 35 and 48 °C. From the results of this study, I argue that by utilizing high ratios of cutaneous EWL to respiratory EWL, doves generate much less metabolic heat at high Tas than passerines. I suggest that larger passerines are better able to tolerate heat than smaller passerines, whereas the opposite is the case in doves. The lack of data from small doves obscured this finding in the conventional and PIC analyses. Further studies on the upper limits to the avian capacity for evaporative cooling and heat tolerance are critical for larger-scale mechanistic modeling of vulnerability to extreme heat events under current and future climate scenarios. / Dissertation (MSc)--University of Pretoria, 2014. / DST/NRF Centre of Excellence at the Percy FitzPatrick Institute (University of Cape Town) / University of New Mexico / Zoology and Entomology / MSc / Unrestricted