O'Connor, R. S. | Search bioRxiv

bioRxiv2018

Avian thermoregulation in the heat: is evaporative cooling more economical in nocturnal birds?

Evaporative cooling is a prerequisite for avian occupancy of hot, arid environments, and is the only avenue of heat dissipation when air temperatures (Ta) exceed body temperature (Tb). Whereas diurnal birds can potentially rehydrate throughout the day, nocturnal species typically forgo drinking between sunrise and sunset. We hypothesized that nocturnal birds have evolved reduced rates of evaporative water loss (EWL) and more economical evaporative cooling mechanisms than those of diurnal species that permit them to tolerate extended periods of intense heat without becoming lethally dehydrated. We used phylogenetically-informed regressions to compare EWL and evaporative cooling efficiency (ratio of evaporative heat loss [EHL] and metabolic heat production [MHP]; EHL/MHP) among nocturnal and diurnal birds at high Ta. We analyzed variation in three response variables: 1) slope of EWL at Ta between 40 and 46{degrees}C, 2) EWL at Ta = 46{degrees}C, and 3) EHL/MHP at Ta = 46{degrees}C. Nocturnality emerged as a weak, negative predictor, with nocturnal species having slightly shallower slopes and reduced EWL compared to diurnal species of similar mass. In contrast, nocturnal activity was positively correlated with EHL/MHP, indicating a greater capacity for evaporative cooling in nocturnal birds. However, our analysis also revealed conspicuous differences among nocturnal taxa. Caprimulgids and Australian-owlet nightjars had shallower slopes and reduced EWL compared to similarly-sized diurnal species, whereas owls had EWL rates comparable to diurnal species. Consequently, our results did not unequivocally demonstrate more economical cooling among nocturnal birds. Owls predominately select refugia with cooler microclimates, but the more frequent and intense heat waves forecast for the 21st century may increase microclimate temperatures and the necessity for active heat dissipation, potentially increasing owls vulnerability to dehydration and hyperthermia.

physiology↗

bioRxiv2017

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 (Te) that markedly differ from air temperature (Ta). Quantifying Te thus becomes important for accurately modeling energy and water balance. We measured Te 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 Te and Ta profiles with evaporative water loss (EWL) data for Rufous-cheeked Nightjars. Between 12:00 and 15:00 hrs, average Te at roost sites varied from 33.1 to 49.9 {degrees}C, whereas at the single nest site Te averaged 51.4 {degrees}C. Average diurnal EWL, estimated using Te, 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 Ta 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.

biophysics↗