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.