Seasonal metabolic variation in two populations of an Afrotropical euplectid bird

Many birds exhibit seasonal phenotypic flexibility in basal metabolic rate (BMR) and summit metabolism (M(sum)), but very little information is available for species from subtropical latitudes or for conspecific populations from sites that vary in climate. We measured body mass (M(b)), BMR, and M(sum) in summer and winter in two populations of the southern red bishop Euplectes orix, a passerine that is widespread in southern Africa. One population occurs at a comparatively warmer coastal site (mean daily minimum air temperature [T(a)] in midwinter, 8.3°C) and the other at a colder inland site (mean daily minimum T(a) in midwinter, -2.8°C). Bishops from both populations significantly increased M(b) in winter. However, seasonal metabolic adjustments differed considerably between the populations. The inland population significantly increased BMR by approximately 58%, mass-specific BMR by approximately 31%, and M(sum) by approximately 15% in winter, although mass-specific M(sum) did not change significantly. In contrast, the coastal population showed no significant seasonal change in BMR and significant winter reductions in mass-specific BMR (~15%), M(sum) (~8%), and mass-specific M(sum) (~15%). The interpopulation differences in the magnitude and direction of seasonal mass-specific BMR changes are qualitatively consistent with global patterns, although the increase shown by the inland population is larger than expected. Our data reveal that avian seasonal metabolic adjustments can vary greatly within subtropical species depending on the climatic conditions experienced by the birds, and our findings reiterate the need to better understand metabolic flexibility in species that inhabit lower latitudes.