Avian thermoregulation in the heat: resting metabolism, evaporative cooling and heat tolerance in Sonoran Desert doves and quail

Abstract

Birds in subtropical deserts face significant thermoregulatory challenges because environmental temperatures regularly exceed avian body temperature. To understand the differing susceptibility of desert birds to increasing temperatures, we examined thermoregulatory performance and estimated heat tolerance limits (HTLs) for three Sonoran Desert nesting bird species -Gambel's quail, mourning doves and white-winged doves. Using flow-through respirometry we measured daytime resting metabolism, evaporative water loss and real-time body temperature at air temperatures (T air ) from 30°C to 66°C. We found marked increases in resting metabolism at the upper critical temperature (T uc ), which was significantly lower in the quail (T air =41.1°C) than in both dove species (T air =45.9-46.5°C). Gambel's quail maintained low resting metabolic rates and low rates of evaporative water loss at their T uc (0.71 W and 1.20 g H 2 O h −1 , respectively), but were more sensitive to increasing air temperature, reaching their HTL at T air of 52°C. Mourning doves and white-winged doves maintained low resting metabolic rates (0.66 and 0.94 W), but higher rates of evaporative water loss (1.91 and 2.99 g H 2 O h −1 ) at their T uc and reached their HTL at T air of 58-60°C. Mass-specific evaporative water loss in white-winged doves (147 g) and mourning doves (104 g) was 45% and 30% greater, respectively, than the rate observed in Gambel's quail (161 g) at T air of 48°C. Higher rates of evaporation and higher T uc made the doves exceptionally heat tolerant, allowing them to maintain body temperatures at least 14°C below air temperatures as high as 60°C (140°F).

Avian thermoregulation in the heat: efficient evaporative cooling allows for extreme heat tolerance in four southern Hemisphere columbids

“…In the present study, T b in birds at their heat tolerance limits (i.e. either escape behaviour or thermal endpoint) was generally consistent with values observed in two columbids and a quail by Smith et al (2015); in all cases, T b exceeded 45°C and/or increased at a rate of >0.1°C min −1 during the last 5 min of measurements. The T a associated with the onset of gular flutter for each bird in the chamber was also recorded.…”

“…Namaqua doves were dissipating ∼470% of their metabolic heat load at T a =60°C, a percentage higher than those recorded for other columbids at the same T a (∼286, 369 and 308% in heat-acclimated rock doves, white-winged doves and mourning doves, respectively; Marder and Arieli, 1988;Smith et al, 2015). We hence conclude that, among columbids, Namaqua doves have an unusually pronounced capacity for evaporative cooling.…”

“…spotted nightjar Eurostopodus argus; Dawson and Fisher, 1969). The skin surface area/mass ratio for Namaqua doves is 25-42% greater than those of the columbids examined by Marder and Arieli (1988) and Smith et al (2015), raising the possibility that the greater surface area/ volume ratio of this species may be a factor contributing to the very rapid rates of cutaneous evaporative heat dissipation and consequently high EHL/MHP ratio. Despite the comparatively large volumes of water necessary for this mechanism of heat dissipation, water storage within the crop (Williams and Koenig, 1980) may make it sustainable over short periods.…”

“…We used the same experimental protocol during which birds were exposed to progressively higher T a in a stepped profile as described by Whitfield et al (2015) and Smith et al (2015), and analyzed our data in the same way as in these studies. Birds were exposed to T a values of 25-40°C in 5°C increments, and T a values of 40-62°C in 2°C increments.…”

“…The goal of the present study, like those of Whitfield et al (2015) and Smith et al (2015), was to quantify upper limits to heat tolerance and evaporative cooling capacity in a manner facilitating direct comparisons of standardized variables among species. The conditions birds experienced during our experimental protocol (rapid increases in T a combined with very low humidity values maintained via high flow rates) are unlikely to directly mirror conditions they routinely experience in natural habitats, although humidity values in desert habitats are often similar to those experienced by birds in our study (see e.g.…”