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

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 21st century could perhaps create thermal conditions under which Rufous-cheeked Nightjars exceed dehydration tolerance limits before the onset of their nocturnal active phase.