Mapping evaporative water loss in desert passerines reveals an expanding threat of lethal dehydration

Abstract

Extreme high environmental temperatures produce a variety of consequences for wildlife, including mass die-offs. Heat waves are increasing in frequency, intensity, and extent, and are projected to increase further under climate change. However, the spatial and temporal dynamics of die-off risk are poorly understood. Here, we examine the effects of heat waves on evaporative water loss (EWL) and survival in five desert passerine birds across the southwestern United States using a combination of physiological data, mechanistically informed models, and hourly geospatial temperature data. We ask how rates of EWL vary with temperature across species; how frequently, over what areas, and how rapidly lethal dehydration occurs; how EWL and die-off risk vary with body mass; and how die-off risk is affected by climate warming. We find that smaller-bodied passerines are subject to higher rates of mass-specific EWL than larger-bodied counterparts and thus encounter potentially lethal conditions much more frequently, over shorter daily intervals, and over larger geographic areas. Warming by 4 °C greatly expands the extent, frequency, and intensity of dehydration risk, and introduces new threats for larger passerine birds, particularly those with limited geographic ranges. Our models reveal that increasing air temperatures and heat wave occurrence will potentially have important impacts on the water balance, daily activity, and geographic distribution of arid-zone birds. Impacts may be exacerbated by chronic effects and interactions with other environmental changes. This work underscores the importance of acute risks of high temperatures, particularly for small-bodied species, and suggests conservation of thermal refugia and water sources.

Keywords: avian ecology; climate change; heat waves; physiological ecology; water balance.

Fig. 1.

Evaporative water loss (EWL) as a percentage of body mass (M_b) for five passerine bird species across a range of elevated chamber T_air. Mean body masses (M_b) are indicated and a linear fit line (P << 0.001) has been added for T_air above T_uc of 40 °C. A small amount of jitter (10% minimum distance between adjacent unique values) has been added to minimize overplotting.

Fig. 2.

Current (1980–2012) and future (2070–2100) number of days between April 1 and September 30 with survival time of ≤5 h across the range (orange lines) of three passerine species in the southwestern United States, arranged in order of increasing M_b. Maps of additional species and for ≤3-h dehydration risk are found in Fig. S1.

Fig. S1.

Current (1980–2012) and future (2070–2100) number of days between April 1 and September 30 with survival times of ≤5 h and ≤3 h across the range (orange lines) of five passerine species in the southwestern United States. Insectivore species (Left) and granivores (Right) are arranged in order of increasing M_b from top to bottom within columns.

Fig. 3.

Percentage area of species ranges in the Southwest study region subject to conditions associated with lethal dehydration under current (Upper) and future 4 °C warmer (Lower) climates for moderate (survival time ≤5 h; Left) and high (survival time ≤3 h; Right) dehydration risk. LEGO, Lesser Goldfinch; HOFI, House Finch; CAWR, Cactus Wren; ABTO, Abert’s Towhee; CBTH, Curve-billed Thrasher. The bottom of box, heavy central line, and top of box indicate areas experiencing ≥10 d⋅y⁻¹, ≥5 d⋅y⁻¹, and ≥1 d⋅y⁻¹, respectively. The width of each box is proportional to the range area of species within the conterminous United States, and abscissa position is determined by body mass.

Fig. 4.

Average days per year with 5-h (Left) and 3-h (Right) lethal dehydration intensities in current and future climates. Averages are calculated over each species’ range within the US hot desert ecoregion, comprising the Mojave, Sonoran, and Chihuahuan deserts (ecoregion data from ref. 39).