Abstract:
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 studieshave revealedgreater 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 KalahariDesert
populations of two Afrotropical passerines, the whitebrowed
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 (Msum; maximum cold-induced resting metabolic
rate) and quantified seasonal fluctuations in air temperature (Ta)
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 to68%highercompared with thesummerBMR. In contrast
to higher-latitude species, Msum was not related to the cold-limit
temperature of birds or to winterminimumTa at the study site, but
interannual variation in BMR andMsum was significantly lower in
seasons with lower food abundance in both WBSWs and SFWs.
Ourdata 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.
