Abstract:
Heterothermy plays an important role in lowering the costs of thermoregulation
in endotherms by reducing water and energy requirements. We tested predictions that birds in
arid habitats should express fine-scale variation in their thermoregulatory patterns as a
function of prevailing climatic conditions. We assessed effects of air temperature (Tair) and
water vapor pressure deficit (D) on body temperature (Tb) in free-living White-browed
Sparrow-Weavers (Plocepasser mahali) during summer in two arid habitats in the Kalahari
Desert, South Africa, using data from a dry period at a hot, desert site (n ¼ 7 birds), and
during a dry period (n¼4 birds) and a wet period (n¼5 birds) at a milder, semi-desert site. The
desert birds maintained a significantly higher set-point Tb (41.58 6 0.28C, mean 6 SD) than
semi-desert birds (40.28 6 0.28C). During the warmest part of day (12:00–18:00 hours), Tb
increased significantly during periods of high Tair and/or high humidity, and mean and
maximum Tb were up to 1.48 and 2.38C, respectively, above normal levels. However, as Tair
increased, birds at the desert site maintained Tb at or below set-point levels for a greater
proportion of the time than birds at the semi-desert site. Birds at the desert site also expressed
a greater magnitude of daily heterothermy (heterothermy index, HI ¼ 2.48 6 0.38C, mean 6
SD) than birds at the semi-desert site: the latter population showed a greater magnitude of
heterothermy during a dry period (HI ¼ 2.18 6 0.38C) than during a wet period (HI ¼ 1.68 6
0.28C). Birds continued foraging throughout the warmest part of the day, despite the fact that
heat dissipation (percentage of time spent panting and wing-spreading) increased significantly
with increasing Tair. Our findings reveal that populations can vary in their thermoregulatory
responses in both space and time and suggest that small changes in Tair can have significant
effects on thermoregulation in free-ranging desert birds, even when Tair , Tb. These data have
important implications for assessing vulnerability of species to climate change, suggesting that
sensitivity should be assessed at the population, rather than species, level.
