Environmental determinants of total evaporative water loss in birds at multiple temperatures

Abstract

Endotherms dissipate heat to the environment to maintain a stable body temperature at high ambient temperatures, which requires them to maintain a balance between heat dissipation and water conservation. Birds are relatively small, contain a large amount of metabolically expensive tissue, and are mostly diurnal, making them susceptible to physiological challenges related to water balance and heat dissipation. We compiled total evaporative water loss (TEWL) measurements for 174 species of birds exposed to different temperatures and used comparative methods to examine their relationships with body size, ambient temperature, precipitation, diet, and diel activity cycle. TEWL in the thermoneutral zone (TNZ) was associated primarily with body mass and activity phase. Larger and more active-phase birds, with their higher metabolic rates, lost more water through evaporation than smaller, resting-phase birds, particularly at higher thermal exposures. However, maximum temperature of the natural habitat became an important determinant of TEWL when birds were exposed to temperatures exceeding the TNZ. Species from hotter climates exhibited higher TEWL. Adaptation to arid climates did not restrict evaporative water loss at thermal conditions within the TNZ, but promoted evaporative water loss at exposures above the TNZ. The TEWL of granivores, which ingest food with low water content, differed little from species with other food habitats under all thermal conditions. The effects of environmental covariates of TEWL were dissimilar across thermal exposures, suggesting no evidence for a tradeoff between water conservation in the TNZ and heat dissipation at exposure to higher temperatures. Thus, birds may be able to acclimate when climate change results in the need to increase heat dissipation due to warming, except perhaps in hot, arid environments where species will need to depend heavily upon evaporative cooling to maintain homeothermy.

Two lines of evidence for physiological control of insensible evaporative water loss by a tiny marsupial

“…Schmidt-Nielsen, 1975;Cooper et al, 2019). Correlations between IEWL and ecological and environmental factors (Williams, 1996;Withers et al, 2006;Van Sant et al, 2012;Song and Beissinger, 2020) indicate selective pressure on IEWL, similar to that observed for body temperature (T b ) and metabolic rate (MR, e.g. basal MR, field MR; Nagy, 1987;McNab, 2003;White and Seymour, 2004;Withers et al, 2006;Withers et al, 2016).…”

Limited heat tolerance in a cold-adapted seabird: implications of a warming Arctic

“…In contrast, heat tolerance limits increased with M b in Australian passerines ( McKechnie et al, 2017 ), but there was no clear relationship in Sonoran passerines ( Smith et al, 2017 ). Body mass was the most important predictor of EWL across 174 bird species, with higher EWL rates in larger birds ( Song and Beissinger, 2020 ); however, smaller passerines experience higher rates of mass-specific EWL rates and have a greater risk of dehydration than larger birds ( Albright et al, 2017 ; McKechnie and Wolf, 2010 ). Larger murres demonstrated steeper increases in RMR and shallower increases in EWL, which clearly influenced EHL/MHP.…”

The thermoregulatory role of relative bill and leg surface areas in a Mediterranean population of Great tit (Parus major)

“…The latter is the only avenue by which birds can maintain normothermic T b when T a exceeds T b (e.g., McKechnie & Wolf, 2019 ) and is an energetically expensive process that is affected by the environmental humidity (e.g., Smith et al, 2015 , van Dyk et al, 2019 ). This implies a critical trade‐off between the need to avoid hyperthermia and the risk of dehydration, particularly in bird species inhabiting hot and arid environments (Boyles et al, 2011 ; Czenze et al, 2020 ; Oswald et al, 2018 ; Smit et al, 2013 ; Song & Beissinger, 2020 ).…”