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

Abstract

26Evaporative cooling is a prerequisite for avian occupancy of hot, arid environments, and is the 27 only avenue of heat dissipation when air temperatures (Ta) exceed body temperature (Tb). 28Whereas diurnal birds can potentially rehydrate throughout the day, nocturnal species typically 29 forgo drinking between sunrise and sunset. We hypothesized that nocturnal birds have evolved 30 reduced rates of evaporative water loss (EWL) and more economical evaporative cooling 31 mechanisms than those of diurnal species that permit them to tolerate extended periods of intense 32 heat without becoming lethally dehydrated. We used phylogenetically-informed regressions to 33 compare EWL and evaporative cooling efficiency (ratio of evaporative heat loss [EHL] and 34 metabolic heat production [MHP]; EHL/MHP) among nocturnal and diurnal birds at high Ta. We 35 analyzed variation in three response variables: 1) slope of EWL at Ta between 40 and 46ºC, 2) 36 EWL at Ta = 46ºC, and 3) EHL/MHP at Ta = 46ºC. Nocturnality emerged as a weak, negative 37 predictor, with nocturnal species having slightly shallower slopes and reduced EWL compared to 38 diurnal species of similar mass. In contrast, nocturnal activity was positively correlated with 39 EHL/MHP, indicating a greater capacity for evaporative cooling in nocturnal birds. However, 40 our analysis also revealed conspicuous differences among nocturnal taxa. Caprimulgids and 41Australian-owlet nightjars had shallower slopes and reduced EWL compared to similarly-sized 42 diurnal species, whereas owls had EWL rates comparable to diurnal species. Consequently, our 43

High air temperatures induce temporal, spatial and social changes in the foraging behaviour of wild zebra finches

“…The importance of water and the physiological constraints of heat were further supported by our observations of birds during the hottest periods of the day, and the hottest days, when we encountered zebra finches sitting in small groups near the water. We found that by the time air temperatures reached 35 °C, a significant proportion of birds were panting, in line with previous studies (Schleucher, 1993;Zann, 1996), as well as wing spreading or fanning (Eto, Withers, & Cooper, 2017;O'Connor et al, 2018;Tieleman & Williams, 1999). Above 41 °C individuals also exhibited two other behaviours, which we were unable to find any reference to, in a heat dissipation context.…”

High air temperatures induce temporal, spatial and social changes in the foraging behaviour of wild zebra finches

“…The importance of water and the physiological constraints of heat were further supported by our observations of birds during the hottest periods of the day, and the hottest days, when we encountered zebra finches sitting in small groups near the water. We found that by the time air temperatures reached 35 °C, a significant proportion of birds were panting, in line with previous studies (Schleucher, 1993;Zann, 1996), as well as wing spreading or fanning (Eto, Withers, & Cooper, 2017;O'Connor et al, 2018;Tieleman & Williams, 1999). Above 41 °C individuals also exhibited two other behaviours, which we were unable to find any reference to, in a heat dissipation context.…”

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).…”

High air temperatures induce temporal, spatial and social changes in the foraging behaviour of wild zebra finches

“…The importance of water and the physiological constraints of heat were further supported by our observations of birds during the hottest periods of the day, and the hottest days, when we encountered zebra finches sitting in small groups near the water. We found that by the time air temperatures reached 35 °C, a significant proportion of birds were panting, in line with previous studies (Schleucher, 1993;Zann, 1996), as well as wing spreading or fanning (Eto, Withers, & Cooper, 2017;O'Connor et al, 2018;Tieleman & Williams, 1999). Above 41 °C individuals also exhibited two other behaviours, which we were unable to find any reference to, in a heat dissipation context.…”

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).…”

High air temperatures induce temporal, spatial and social changes in the foraging behaviour of wild zebra finches

“…The importance of water and the physiological constraints of heat were further supported by our observations of birds during the hottest periods of the day, and the hottest days, when we encountered zebra finches sitting in small groups near the water. We found that by the time air temperatures reached 35 °C, a significant proportion of birds were panting, in line with previous studies (Schleucher, 1993;Zann, 1996), as well as wing spreading or fanning (Eto, Withers, & Cooper, 2017;O'Connor et al, 2018;Tieleman & Williams, 1999). Above 41 °C individuals also exhibited two other behaviours, which we were unable to find any reference to, in a heat dissipation context.…”

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).…”