Diurnal body temperature patterns in free‐ranging populations of two southern African arid‐zone nightjars

Endotherms allocate large amounts of energy and water to the regulation of a precise body temperature (T_b), but can potentially reduce thermoregulatory costs by allowing T_b to deviate from normothermic levels. Many data on heterothermy at low air temperatures (T_a) exist for caprimulgids, whereas data on thermoregulation at high T_a are largely absent, despite members of this taxon frequently roosting and nesting in sites exposed to high operative temperatures. We investigated thermoregulation in free‐ranging rufous‐cheeked nightjars Caprimulgus rufigena and freckled nightjars Caprimulgus tristigma in the southern African arid zone. Individuals of both species showed labile T_b fluctuating around a single modal T_b (T_b‐mod). Average T_b‐mod was 39.7°C for rufous‐cheeked nightjars and 39.0°C for freckled nightjars. In both species, diurnal T_b increased with increasing T_a. At T_a ≥ 38°C, rufous‐cheeked nightjar mean T_b increased to 42°C, equivalent to 2.3°C above T_b‐mod. Under similar conditions, freckled nightjar T_b was on average only 1.1°C above T_b‐mod, with a mean T_b of 40.0°C. Freckled nightjars are one of the most heterothermic caprimulgids investigated to date, but our data suggest that during hot conditions this species maintains T_b within a narrow range above T_b‐mod, possibly reflecting an evolutionary tradeoff between decreased thermal sensitivity to lower T_b but increased sensitivity to high T_b. These findings reveal how general thermoregulatory patterns at similar T_a can vary even among closely related species.