Extreme operative temperatures in exposed microsites used by roosting Rufous-cheeked Nightjars (Caprimulgus rufigena): implications for water balance under current and future climate conditions

Abstract

Nocturnally active birds roosting in exposed microsites can experience operative temperatures (Te) that markedly differ from air temperature (Ta). Thus, quantifying Te becomes important for accurately modeling energy and water balance. We measured Te at roost and nest sites used by Rufous-cheeked Nightjars (Caprimulgus rufigena A. Smith, 1845) (mean body mass = 57.1 g) with three-dimensionally printed models covered with the plumage of a bird. Additionally, we estimated site-specific diurnal water requirements for evaporative cooling by integrating Te and Ta profiles with evaporative water loss (EWL) data for Rufous-cheeked Nightjars. Between the hours of 12:00 and 15:00, representing maximum solar radiation, mean Te at roost sites varied from 33.1 to 49.9 °C, whereas at the single nest site, Te averaged 51.4 °C. Mean diurnal EWL, estimated using Te, ranged from 2.8 to 10.5 g among roosts, values 1.2- and 3.6-fold greater, respectively, than Ta estimates. At the nest site, total EWL estimated using Te was 11.3 g, 4.0-fold greater than the corresponding estimate based on Ta. Consequently, Rufous-cheeked Nightjars can experience EWL potentially approaching their limits of dehydration tolerance. In the absence of microsite changes, climate change during the 21st century could perhaps create thermal conditions under which Rufous-cheeked Nightjars exceed dehydration tolerance limits before the onset of their nocturnal active phase.