Roosting in exposed microsites by a nocturnal bird, the rufous-cheeked nightjar: implications for water balance under current and future climate conditions

Abstract

Nocturnally active birds roosting in exposed diurnal microsites with intense solar radiation can experience operative temperatures (T_e) that markedly differ from air temperature (T_a). Quantifying T_e thus becomes important for accurately modeling energy and water balance. We measured T_e at roost and nest sites used by Rufous-cheeked Nightjars (Caprimulgus rufigena) with three-dimensionally printed biophysical models covered with the integument and plumage of a bird. Additionally, we estimated site-specific diurnal water requirements for evaporative cooling by integrating T_e and T_a profiles with evaporative water loss (EWL) data for Rufous-cheeked Nightjars. Between 12:00 and 15:00 hrs, average T_e at roost sites varied from 33.1 to 49.9 °C, whereas at the single nest site T_e averaged 51.4 °C. Average diurnal EWL, estimated using T_e, was as high as 10.5 and 11.3 g at roost and nest sites, respectively, estimates 3.8- and 4.0-fold greater, respectively, than when calculated with T_a profiles. These data illustrate that under current climatic conditions, Rufous-cheeked Nightjars can experience EWL potentially approaching their limits of dehydration tolerance. In the absence of microsite changes, climate change during the 21^st century could perhaps create thermal conditions under which Rufous-cheeked Nightjars exceed dehydration tolerance limits before the onset of their nocturnal active phase.