Abstract:
Many birds exhibit seasonal phenotypic flexibility in basal metabolic
rate (BMR) and summit metabolism (Msum), 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 (Mb), BMR, and Msum 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 [Ta] inmidwinter,
8.3 C) and the other at a colder inland site (mean daily
minimum Ta in midwinter, 2.8 C). Bishops from both populations
significantly increased Mb 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 Msum by approximately 15% in winter, although massspecific
Msum 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%), Msum (∼8%), and mass-specific Msum (∼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.
