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

Abstract

Endotherms allocate large amounts of energy and water to the regulation of a precise body temperature (Tb), but can potentially reduce thermoregulatory costs by allowing Tb to deviate from normothermic levels. Many data on heterothermy at low air temperatures (Ta) exist for caprimulgids, whereas data on thermoregulation at high Ta 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 Tb fluctuating around a single modal Tb (Tb‐mod). Average Tb‐mod was 39.7°C for rufous‐cheeked nightjars and 39.0°C for freckled nightjars. In both species, diurnal Tb increased with increasing Ta. At Ta ≥ 38°C, rufous‐cheeked nightjar mean Tb increased to 42°C, equivalent to 2.3°C above Tb‐mod. Under similar conditions, freckled nightjar Tb was on average only 1.1°C above Tb‐mod, with a mean Tb 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 Tb within a narrow range above Tb‐mod, possibly reflecting an evolutionary tradeoff between decreased thermal sensitivity to lower Tb but increased sensitivity to high Tb. These findings reveal how general thermoregulatory patterns at similar Ta can vary even among closely related species.

Interspecific variation in avian thermoregulatory patterns and heat dissipation behaviours in a subtropical desert

“…The few studies that have examined diurnal Tb patterns in free-ranging birds during hot weather typically focus on just one or two species (Smit et al 2013, O'Connor et al 2017. These studies confirm that Tb patterns can differ substantially between free-ranging individuals and individuals held in respirometry chambers (O'Connor et al 2017). Yet, despite the importance of thermoregulation and associated trade-offs in the ecology of desert birds, very little is known about interspecific differences in thermoregulatory patterns in free-ranging birds (Smit et al 2013) or how these relate to species traits, including variation in the temperature-dependence of HDB.…”

“…However, laboratory studies may not accurately reflect thermoregulation under natural conditions, as ambient conditions are controlled, and activity restricted (Smit et al 2013). The few studies that have examined diurnal Tb patterns in free-ranging birds during hot weather typically focus on just one or two species (Smit et al 2013, O'Connor et al 2017. These studies confirm that Tb patterns can differ substantially between free-ranging individuals and individuals held in respirometry chambers (O'Connor et al 2017).…”

Heterothermy in a Small Passerine: Eastern Yellow Robins Use Nocturnal Torpor in Winter

“…While this evidence supports torpor use in passerine species, it is important to evaluate these functions in the wild. Captive conditions do not represent the complexity of the thermal conditions and ecological complexity (e.g., food abundance, predation risk) experienced in the wild, and laboratory studies may therefore underestimate the use of torpor by wild animals (Geiser et al, 2000;O'connor et al, 2017).…”

Environmental and ecological correlates of avian field metabolic rate and water flux

“…How animals allocate energy to maintenance, reproduction and growth provides a basis for understanding life history evolution and physiological adaptation. Throughout its life in the wild, an animal is exposed to a variety of physical and biological environments, and must balance energy intake, thermoregulation and water conservation according to the ambient conditions it encounters (Greenberg, Cadena, Danner, & Tattersall, ; O'Connor, Brigham, & McKechnie, ; Williams, ) . The energy and water budget of an animal in the field is represented by its field metabolic rate (FMR) and field water flux (FWF) respectively, which are often measured using the doubly labelled water method (Speakman & Hambly, ).…”