Predicted increases in the frequency, intensity and duration of heat waves in the desert of the southwestern United States put songbirds at increased risk of death from dehydration and mass mortality, new study finds .

The researchers used hourly temperature maps and other data produced by the North American Land Data Assimilation System (NLDAS) – a modeling effort of the earth’s surface maintained by NASA and other organizations – as well as physiological data. to study how the rates of evaporative water loss in response to high temperatures varied between five species of birds with different body masses. Using this data, they were able to map the potential effects of current and future heat waves on the risk of deadly dehydration for songbirds in the southwest and how quickly dehydration can occur in each species.

The researchers focused on five species of songbirds commonly found in the southwestern desert: the goldfinch, house finch, cactus wren, Abert’s tohi, and curved-billed mockingbird. .

Under projected conditions where temperatures rise by 4 degrees Celsius (7 degrees Fahrenheit), which is consistent with some summer warming scenarios by the turn of the century, heat waves will occur more often, become hotter and s ‘will expand geographically to the point where all five species will be at greater risk of fatal dehydration.

Birds are sensitive to heat stress in two ways, said co-author Blair Wolf, professor of biology at the University of New Mexico. With funding from the National Science Foundation, Wolf studied heat tolerance for each of the five species in the study as well as other bird species in Australia and South Africa. “When it’s really hot, they just can’t evaporate enough water to stay cool, so they overheat and die from heat stroke,” he said. “In other cases, the high rates of evaporative water loss needed to stay cool deplete their body water supplies to lethal levels and birds die of dehydration. It’s the stressor we’re on. we are focused in this study. “

What’s going on is around 40 degrees Celsius [104 degrees Fahrenheit], these songbirds begin to gasp, which increases the rate of water loss very quickly, said co-author Alexander Gerson, assistant professor of biology at the University of Massachusetts-Amherst. At the time of the study, he was working with Wolf as a postdoctoral researcher at the University of New Mexico. He added: “Most animals can only tolerate water loss that results in a loss of 15 or 20 percent of body mass before dying. Thus, an animal experiencing peak temperatures on a hot day in summer, without access to water, not more than a few hours. “

As expected, they found that small species are particularly susceptible to deadly dehydration because they lose water at a proportionately higher rate. For example, at 50 degrees Celsius [122 degrees Fahrenheit], the small goldfinch and house finch lose 8 to 9 percent of their body mass due to evaporative water loss per hour, while the larger curved-billed mockingbird only loses about 5 percent of its body mass. mass per hour. By the turn of the century, the number of days in the southwestern desert where fatal dehydration poses a high risk for the goldfinch increased from 7 to 25 days per year. For larger species, those days will also increase, but remain rare.

Despite their physiological disadvantage, domestic finches and goldfinches might actually fare comparatively better, the researchers noted, as they can survive in a number of ecosystems and have a wider range. But desert specialists such as the Bent-billed Thrasher and Abert’s tohi have more specific habitat requirements and therefore have a more limited range, limited to the United States primarily to the hot deserts of the southwest. This means that a greater proportion of their population is at risk of fatal dehydration when sufficiently severe heat waves occur.

“When you find yourself in a situation where the majority of the range is affected, that’s where we start to worry more about what we see,” said lead author Tom Albright of the University of Nevada, Reno, noting that this increases the risk of fatal dehydration affecting a large part of the population.

According to the researchers, given this warming scenario, climate refuges – microclimates such as mountain peaks, trees and shady lakes that allow songbirds to cool down to safe levels – could develop. be very important in management plans for certain vulnerable species. “By using this kind of data, managers identifying the best refugia can get a better idea of ​​the temperature profile that will suit these birds,” Gerson said.