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Extreme Heat: Assessing Impacts of Heat and Microclimate on Birds of the Sonoran Desert
AuthorJacobs, Devin L.
AdvisorAlbright, Thomas P
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At high temperatures, birds can face increased levels of stress which may ultimately impose fitness costs. Average air temperatures across much of the United States are expected to exceed thermal tolerance limits of many birds over the next 100 years. To reduce thermal pressures, birds may be able to use various physiological heat dissipation behaviors (HDB), and/or select vegetated microsites to perch within that may provide cooler environmental temperatures than the ambient surroundings. Although, both microsite selection and heat dissipating behaviors have been shown to impact bird survival during heat waves, the relative costs and benefits of performing each of these actions requires further exploration and quantification. The purpose of this study is to detail the thermal heterogeneity available in a Sonoran Desert ecosystem due to microclimatic variation, and examine the ways in which different birds use this thermal heterogeneity in combination with heat dissipating behaviors. In chapter one, I assess differences in air temperatures recorded across a range of vegetated microsite densities, and use linear models to examine how air temperature differences respond to varying weather and vegetation variables. Temperatures within densely vegetated microsites increased at a slower rate than the ambient surroundings (.97 °C for every 1 °C increase in ambient temperature), and led to an even greater proportional reduction in exposure to cumulative degree-hours above 40 °C. Differences between microsite temperatures and estimated ambient air temperatures provided by a gridded temperature product highlight the need to account for microclimatic variation when estimating thermal impacts over large areas. In chapter 2, to examine the ways in which birds use variations in their thermal environment at high temperatures, I quantify preference shifts in perch locations across time with χ2 tests, and use logistic models to examine the combined impact of microsite selections and weather conditions on different birds’ use of physiological heat dissipating behaviors. The strength of perch preference shifts varied strongly between species, and birds were observed performing HDBs more often within more heavily vegetated microsites. This implies that necessity and/or benefit of using microsite selection and HDBs may vary between species with different physiological characteristics, and that vegetated microsites are used to avoid unnecessary thermal costs at high temperatures. These studies illustrate that predictions of the thermal impacts to birds from heat waves should incorporate the range of temperatures available due to microclimatic variation, and should consider the behavioral options and preferences of birds under different weather conditions.