High temperatures, evaporative water loss, and desert avifauna: Mapping dehydration in the Cactus Wren

Background/Question/Methods

High temperature extremes have a variety of direct and indirect effects on animals, including large scale die-offs affecting thousands of individuals.  As the frequency and severity of high temperature extremes are increasing, it is important to understand where and when die-offs can be expected to occur.  Dehydration is a major mechanism of heat-related mortality in birds and other endotherms.  Here, we model and map the dynamics of 1979-2012 high temperature extremes and evaporative water loss rates producing potentially lethal dehydration levels for the Cactus Wren (Campylorhynchus brunneicapillus), a common desert bird, over the Southwest US portion of its range.  Evaporative water loss (EWL) across a range of high temperatures was measured in heat-acclimated Cactus Wrens captured in the field.  We used the North American Land Data Assimilation System dataset to obtain an hourly estimates of temperatures with a 14-km spatial grain. Assuming lethal dehydration occurs when water loss reaches 15% of body weight, we then produced maps of total daily EWL and time to lethal dehydration during the 99^thpercentile of hot days in the record.  We repeated this with a 4 °C warming scenario to account for expected climate change during the century. 

Results/Conclusions

Measured EWL rates for the Cactus Wren produced potentially lethal dehydration within five hours, as temperatures increased from 40 °C to 48 °C. Such conditions occurred frequently in much of its range.   For example, on 7 July 2012, potentially lethal conditions (> 40 °C for more than five hours) occurred in 83 % (2094 km²) of the Kofa National Wildlife Refuge Refuge, an area with extensive habitat in Arizona. Temperature maps also indicate substantial spatial variation in EWL, suggesting a potential for regional-scale thermal refugia.  However, increasing maximum temperatures by 4 °C  greatly expanded the number of potentially lethal days and reduced the availability of thermal refugia.  These results illustrate the potential of dehydration to produce large-scale die offs of the Cactus Wren under both current and future climates.  This analysis did not account for the bird’s ability to use fine-scale topographic- and vegetation-mediated refugia, which may retard EWL enough to avoid lethal dehydration levels.  On the other hand, smaller birds will have greater problems maintaining water balance under these conditions resulting in shifts in community composition.  We are currently expanding analysis to account for different evaporative mechanisms, body sizes, and fine-scale thermal refugia.