Physiological variation in amethyst sunbirds (Chalcomitra amethystina) over an altitudinal gradient: a seasonal comparison.

Southern Africa is characterised by an unpredictable environment with daily and seasonal temperature fluctuations, thus posing challenging thermal conditions and increased energetic stress for endothermic vertebrates. Amethyst Sunbirds (Chalcomitra amethystina) are relatively large African sunbirds (15 g). They are considered non-migratory and thus have to cope with the temperature changes and physiological stresses a new season brings. This study compared altitudinal subpopulations and the seasonal shifts in metabolic parameters between and within the subpopulations in metabolic parameters. Amethyst Sunbirds were caught in summer and winter at three different altitudinal subpopulations; Underberg (1555 m asl), Howick (1075 m asl) and Oribi Gorge (541 m asl). Upon capture, metabolic rates of the sunbirds were measured indirectly by quantifying oxygen consumption ((.)VO2) using flow through respirometry, at 5 and 25 degrees C. Birds then underwent a 6-week acclimation period at 25 degrees C on a 12 L: 12D cycle. (.)VO2 was measured post-acclimation at 8 different temperatures (15, 5, 10, 20, 30, 28, 25 and 33 degrees C), which were ordered randomly in the experimental protocol to avoid acclimation bias. Experiments were repeated for a winter and summer season. In general, Amethyst Sunbird subpopulations from Underberg and Howick showed higher post-acclimation resting metabolic rates per temperature in winter than in summer trials. Underberg and Howick subpopulations respectively showed a significant difference between summer and winter (.)VO2 at 5 and 10 degrees C. Thermal neutral zones of all of the subpopulations of sunbirds shifted between winter and summer. Post-acclimation basal metabolic rate of sunbirds decreased significantly by 38.8% from winter to summer for the Underberg subpopulation, increased by 44.8% for the Howick subpopulation and did not change significantly for the Oribi Gorge subpopulation (5.8% decrease).