Amethyst Sunbirds (Chalcomitra amethystina) occur over an altitudinal gradient within KwaZulu-Natal, South Africa, from the Drakensberg mountain range to the coast. Sunbirds were caught in summer (November-December 2006) at three altitudinal locations within KwaZulu-Natal; Underberg (1553 m above sea level (asl)), Howick (1075 m asl) and Oribi Gorge (541 m asl). Summer oxygen consumption (VO 2 ) was measured pre-acclimation at 5 and 25 1C. After post-acclimation to 25 1C and 660 m for 6 weeks, VO 2 measurements were taken at 8 different temperatures (15, 5, 10, 20, 30, 28, 25 and 33 1C). Summer resting metabolic rates (RMR) differed significantly between altitudinal subpopulations of Amethyst Sunbirds. Summer basal metabolic rate was observed to decrease as altitude decreased. Differences between pre-acclimation RMR (representing natural acclimatization to summer conditions in the field) and post-acclimation RMR (to 25 1C at 660 m asl for 6 weeks) were large. In particular, Sunbirds from the Underberg subpopulation increased RMR by 80.9% at 5 1C and 85.2% at 25 1C postacclimation. This summer research complemented an earlier study on altitudinal variation in Amethyst Sunbirds in winter. It again emphasized the need to acknowledge altitudinal differences between subpopulations and not just use species means, as species means do not fully incorporate the effect of phenotypic plasticity/flexibility.