Avian seasonal metabolic variation in a subtropical desert: basal metabolic rates are lower in winter than in summer

P>Most small birds inhabiting temperate latitudes in the Holarctic increase basal metabolic rate (BMR) in winter, a pattern thought to reflect the up-regulation of metabolic machinery required for enhanced winter cold tolerance. In contrast, patterns of seasonal BMR variation in birds inhabiting subtropical latitudes are largely unknown. In this study, we investigate seasonal BMR changes in species from subtropical latitudes, and analyse global variation in the direction and magnitude of these responses. We estimated winter and summer BMR in five species resident in the Kalahari Desert, using flow-through respirometry to measure O-2 consumption and CO2 production in birds held overnight in a field laboratory. In all five species, mass-specific BMR was significantly lower in winter than in summer, with mean reductions of 23% in African scops-owls (Otus senegalensis), 30% in pearl-spotted owlets (Glaucidium perlatum), 35% in fork-tailed drongos (Dicrurus adsimilis), 29% in crimson-breasted shrikes (Laniarius atrococcinneus), and 17% in white-browed sparrow-weavers (Plocepasser mahali). An analysis of global variation in seasonal BMR changes reveals that their magnitude and direction vary with latitude, ranging from pronounced winter increases at high latitudes where winters are extremely cold, to the opposite pattern in warmer, subtropical environments. Our empirical results for five species, taken together with the analysis of global variation, are consistent with the hypothesis that winter metabolism in subtropical environments is driven primarily by the need for energy and/or water conservation rather than cold tolerance.