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 (Tb-mod). Average Tb-mod was 39.7 degrees C for rufous-cheeked nightjars and 39.0 degrees C for freckled nightjars. In both species, diurnal T-b increased with increasing T-a. At T-a 38 degrees C, rufous-cheeked nightjar mean T-b increased to 42 degrees C, equivalent to 2.3 degrees C above Tb-mod. Under similar conditions, freckled nightjar T-b was on average only 1.1 degrees C above Tb-mod, with a mean T-b of 40.0 degrees 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 Tb-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.