A new study of songbird dehydration and survival risk during heat waves in the desert Southwest suggests that some birds are at risk of lethal dehydration and mass die-offs when water is scarce, and the risk is expected to increase as climate change advances.
Using physiological data, hourly temperature maps, and modeling, researchers from the universities of Nevada-Reno, New Mexico, and Massachusetts-Amherst investigated how rates of evaporative water loss varied in five bird species with varied body mass. They mapped potential effects of current and future heat waves on lethal dehydration risk for songbirds in the Southwest and how rapidly this can occur in each species. Details were published this week in the Proceedings of the National Academy of Sciences.
The scientists looked at Lesser Goldfinch, House Finch, Cactus Wren, Abert’s Towhee, and Curve-billed Thrasher, representing “a wide range of body sizes,” notes co-author Alexander Gerson, of Massachusetts-Amherst. He and his colleagues found that smaller species lose water faster than larger birds, making them particularly susceptible to lethal dehydration.
“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,” they state. “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.”
“Birds are susceptible to heat stress in two ways,” says Blair Wolf of the University of New Mexico. “When it’s really hot, they simply can’t evaporate enough water to stay cool, overheat and die of heat stroke. In other cases, the high rates of evaporative water loss needed to stay cool deplete their body water pools to lethal levels, and birds die of dehydration. This is the stressor we focused on in this study.”
He adds, “These estimates suggest that some regions of the desert will be uninhabitable for many species in the future, and that future high-temperature events could depopulate whole regions.”
“Most animals can only tolerate water losses that result in 15 or 20 percent loss of body mass before they die,” Gerson adds. “So an animal experiencing peak temperatures during a hot summer day, with no access to water, isn’t going to make it more than a few hours. Once we have these types of profiles for a number of different species, we can determine differential survival rates, which will then drive differences in the overall avian community structure.”
The news does not look good for some species, the biologist acknowledges, “but this study will give us a new tool to try to inform our conservation efforts to try to save these species, or at least understand the impact better on the overall ecosystem. You have to understand the severity of the problem before you can do anything about it.”
One message for conservation is that climate refugia may become increasingly important. Further understanding of microclimates will help: mountaintops, trees, and washes with shade might be very important in management plans for certain vulnerable species. Birds with a wider range such as House Finch and Lesser Goldfinch might fare better, Gerson notes, because they can survive in a number of ecosystems. But specialists such as Curve-billed Thrasher and Abert’s Towhee have more specific habitat needs and face higher risk.
“Using this type of data, managers identifying the best refugia can have a better idea of the temperature profile that will be suitable for these birds,” he says.
Tom Albright, of the University of Nevada-Reno, says the work “shows that in these hot desert systems for these species, we have a potentially devastating mechanism that can lead to die-offs for some species.”
The authors point out that their work is part of a larger effort looking at the biology of birds in the hottest places on Earth related to a real, current threat of massive avian die-offs occurring now in Australia and South Africa, for example.
Read the abstract
Thomas P. Albright, Denis Mutiibwa, Alexander. R. Gerson, Eric Krabbe Smith, William A. Talbot, Jacqueline J. O’Neill, Andrew E. McKechnie, and Blair O. Wolf. Mapping evaporative water loss in desert passerines reveals an expanding threat of lethal dehydration. PNAS 2017, published February 13, 2017, doi:10.1073/pnas.1613625114. Abstract.
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