Meeting Abstract
Endotherms expend large amounts of energy and water to maintain a preferred body temperature (Tb), but can potentially alleviate 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 the propensity for these birds to roost and nest in thermally exposed sites where operative temperatures can approach 60 °C. We investigated thermoregulatory patterns in free-ranging Rufous-cheeked Nightjars (Caprimulgus rufigena) and Freckled Nightjars (Caprimulgus tristigma) in the southern African arid zone. Individuals of both species showed a 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, at a mean Tb of 40.0 °C. Freckled Nightjars are one of the most heterothermic caprimulgids, but our data suggest that during hot conditions this species maintains Tb within a narrow range above Tb-mod, possibly reflecting an evolutionary trade-off between decreased thermal sensitivity to lower Tb but increased thermal sensitivity to high Tb. Our results reveal how general thermoregulatory patterns at similar Ta can vary even among closely related species and raise important questions regarding the coadaptation between thermoregulation and thermosensitivity among highly heterothermic groups.
