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

Abstract

Evaporative cooling is a prerequisite for avian occupancy of hot, arid environments, and is the only avenue of heat dissipation when air temperatures () exceed body temperature (). 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 compared with diurnal species, permitting nocturnal species 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 We analyzed variation in three response variables: (1) slope of EWL at between 40 and 46°C, (2) EWL at =46°C and (3) EHL/MHP at=46°C. Nocturnality emerged as a weak, negative predictor, with nocturnal species having slightly shallower slopes and reduced EWL compared with 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 with similarly sized diurnal species, whereas owls had EWL rates comparable to those of 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.

Environmental conditions lead to shifts in individual communication, which can cause cascading effects on soundscape composition

“…Birds singing under increasingly arid conditions will not only simultaneously face poorer song transmission and territorial resource qualities, but the cost of singing itself will also increase, as the individual must shift allocations of time and energy resources away from singing (Reid, 1987 ; Zollinger & Brumm, 2015 ) and toward thermoregulatory and foraging behavior (du Plessis et al, 2012 ; Funghi et al, 2019 ; Gil & Gahr, 2002 ). Furthermore, aridity likely increases the water costs for singing birds due to accelerated evaporative water loss; singing exposes the high moisture gradient between the bird's respiratory tract and surrounding dry air, causing water loss to the environment (O'Connor et al, 2018 ; Ward et al, 2003 ; Ward & Slater, 2005 ). To avoid dehydration, birds will need to exhibit behavioral flexibility to sing at low dehydration conditions (Ducatez et al, 2020 ) or increase their rate of drinking (Czenze et al, 2020 ).…”

Environmental conditions lead to shifts in individual communication, which can cause cascading effects on soundscape composition

“…Birds singing under increasingly arid conditions will not only simultaneously face poorer song transmission and territorial resource qualities, but the cost of singing itself will also increase, as the individual must shift allocations of time and energy resources away from singing (Reid, 1987 ; Zollinger & Brumm, 2015 ) and toward thermoregulatory and foraging behavior (du Plessis et al, 2012 ; Funghi et al, 2019 ; Gil & Gahr, 2002 ). Furthermore, aridity likely increases the water costs for singing birds due to accelerated evaporative water loss; singing exposes the high moisture gradient between the bird's respiratory tract and surrounding dry air, causing water loss to the environment (O'Connor et al, 2018 ; Ward et al, 2003 ; Ward & Slater, 2005 ). To avoid dehydration, birds will need to exhibit behavioral flexibility to sing at low dehydration conditions (Ducatez et al, 2020 ) or increase their rate of drinking (Czenze et al, 2020 ).…”

Environmental conditions lead to shifts in individual communication, which can cause cascading effects on soundscape composition

“…Birds singing under increasingly arid conditions will not only simultaneously face poorer song transmission and territorial resource qualities, but the cost of singing itself will also increase, as the individual must shift allocations of time and energy resources away from singing (Reid, 1987 ; Zollinger & Brumm, 2015 ) and toward thermoregulatory and foraging behavior (du Plessis et al, 2012 ; Funghi et al, 2019 ; Gil & Gahr, 2002 ). Furthermore, aridity likely increases the water costs for singing birds due to accelerated evaporative water loss; singing exposes the high moisture gradient between the bird's respiratory tract and surrounding dry air, causing water loss to the environment (O'Connor et al, 2018 ; Ward et al, 2003 ; Ward & Slater, 2005 ). To avoid dehydration, birds will need to exhibit behavioral flexibility to sing at low dehydration conditions (Ducatez et al, 2020 ) or increase their rate of drinking (Czenze et al, 2020 ).…”

Environmental conditions lead to shifts in individual communication, which can cause cascading effects on soundscape composition

“…Birds singing under increasingly arid conditions will not only simultaneously face poorer song transmission and territorial resource qualities, but the cost of singing itself will also increase, as the individual must shift allocations of time and energy resources away from singing (Reid, 1987 ; Zollinger & Brumm, 2015 ) and toward thermoregulatory and foraging behavior (du Plessis et al, 2012 ; Funghi et al, 2019 ; Gil & Gahr, 2002 ). Furthermore, aridity likely increases the water costs for singing birds due to accelerated evaporative water loss; singing exposes the high moisture gradient between the bird's respiratory tract and surrounding dry air, causing water loss to the environment (O'Connor et al, 2018 ; Ward et al, 2003 ; Ward & Slater, 2005 ). To avoid dehydration, birds will need to exhibit behavioral flexibility to sing at low dehydration conditions (Ducatez et al, 2020 ) or increase their rate of drinking (Czenze et al, 2020 ).…”