Facultative heterothermy, or torpor, has been demonstrated in different clades of the Apodidae, proposed as a mechanism to reduce energetic demand in response to different physiological cues. In a small tropical Southeast Asian swiftlet species, the Silver-rumped Spinetail (Rhaphidura leucopygialis), we measured the facultative heterothermic response in a pair of experiments. In the first experiment, the mean body temperature of birds immediately after capture was ~38.9°C. Individuals exposed to consistent ambient temperatures of 28.0°C reduced mean body temperatures over time, and these reductions were greater in individuals kept in the dark than in those that were exposed to light during the trial. The second experiment consisted of respirometry trials measuring rates of carbon dioxide production along with internal body temperature in relation to decreasing ambient temperatures. Two different responses to the respirometry trials emerged: Some individuals modulated body temperature in relation to changes in ambient temperature, and other individuals maintained body temperature (~31.6°C) independent of ambient temperature. Our results suggest considerable flexibility among individual heterothermic responses in R. leucopygialis. Our results are in agreement with other studies on swifts, suggesting that these alternative modes for energy conservation are likely context-dependent and based upon individual body condition, life stage, and annual cycle.
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Vol. 132 • No. 3