Seasonal Metabolic Acclimatization Varies in Direction and Magnitude among Years in Two Arid-Zone Passerines

Adaptive plasticity in avian thermal physiology is increasingly apparent, with a well-studied example being metabolic upregulation during cold winters in small birds inhabiting temperate and boreal latitudes. Recent studies have revealed greater variation in the direction and magnitude of seasonal metabolic adjustments among subtropical/tropical birds experiencing milder winters compared with higher-latitude counterparts, suggesting that patterns could vary among years within populations. We quantified seasonal metabolic variation (summer vs. winter) in Kalahari Desert populations of two Afrotropical passerines, the white-browed sparrow-weaver (WBSW; Plocepasser mahali; ∼40 g) and the scaly-feathered weaver (SFW; Sporopipes squamifrons; ∼10 g) over subsequent years (2014-2017). We used flow-through respirometry to measure basal metabolic rate (BMR) and summit metabolism (M_sum; maximum cold-induced resting metabolic rate) and quantified seasonal fluctuations in air temperature (T_a) and food abundance (arthropod and grass seed abundance) at the study site. Our data reveal that the direction and magnitude of seasonal metabolic acclimatization vary among years in both species, with the winter BMR of WBSWs ranging from ∼20% lower to 68% higher compared with the summer BMR. In contrast to higher-latitude species, M_sum was not related to the cold-limit temperature of birds or to winter minimum T_a at the study site, but interannual variation in BMR and M_sum was significantly lower in seasons with lower food abundance in both WBSWs and SFWs. Our data support the idea that patterns of seasonal acclimatization are more variable in birds from lower latitudes and that there is considerable phenotypic flexibility in avian thermal physiology.