The Mojave Desert, like many deserts across the world, is getting hotter and drier. Over the last century, it’s warmed by about 3.6 degrees Fahrenheit (2 degrees Celsius), and its already sparse rainfall has declined by 20 percent in some areas. Even before modern climate change, the Mojave’s birds lived at extremes; many desert birds have evolved special drought adaptations to save water. Now they’re facing conditions that exceed their physiological limits, according to new research published today in Science.
The study accounts for changes in the Mojave’s wildlife over the past century using data from the Grinnell Resurvey Project. The research, led by ornithologist Steve Beissinger at the University of California, Berkeley, retraces the steps of famed naturalist Joseph Grinnell and resurveys the hundreds of sites where he and his field staff counted birds and mammals across California between 1908 and 1968. (Grinnell died in 1939.) By comparing the new data with the old, Beissinger and his team can analyze how bird populations have changed since Grinnell's observations.
According to the resurvey data, the Mojave’s small mammal populations are stable. At 90 study sites across the desert, only three lost mammal species; on average, each site lost two and gained two species. Birds, however, saw drastic declines. Bird diversity went down at 55 out of 61 sites, each losing an average of 18 species. That’s a 42 percent decline in bird diversity across the Mojave, and mostly on protected land. Species that experienced the biggest declines include raptors like American Kestrel, Prairie Falcon, and Turkey Vulture; insectivores like White-throated Swift, Western Kingbird, and Violet-green Swallow; and birds lacking desert adaptations like Northern Mockingbird, Western Wood-Pewee, and Chipping Sparrow. The loss is so catastrophic that scientists have deemed it a “collapse” of the Mojave’s birdlife.
“The entire community of small mammals appears to be unaffected by climate change. That means nothing major happened with the sources of food,” says lead author Eric Riddell, Beissinger’s former postdoc who recently opened his own lab at Iowa State University. “This isn’t an entire ecosystem collapse. It’s the collapse of birds.”
Riddell is an ecophysiologist. He studies how animals’ basic bodily functions respond to changes in their environment. So when he sees these results—that birds are struggling to adapt to the hotter, drier conditions and mammals are not—he wants to know: Why?
For his role in the Grinnell Resurvey Project, instead of camping in the field to repeat century-old surveys, Riddell camped out in UC Berkeley’s Museum of Natural History. He took measurements of hundreds of bird and mammal specimens to create a profile of how each species exchanges heat with its environment. For birds, those measurements included size and shape, body mass, plumage depth, and length of the feather fibers. “You can use those properties to determine the probability that a ray of sunlight penetrates through the plumage,” he says. Essentially, when the sun is shining in the Mojave Desert, how much of that light reflects off a bird and how much is absorbed into its body as heat?
To avoid overheating, a bird needs to get rid of any excess heat taken in. When humans overheat, we sweat to cool off—our body expels water onto our skin, and when that water evaporates, it takes heat with it. The process is called evaporative cooling, and it’s how we avoid major health issues like heat stroke or heat death.
“Humans are really, really good at cooling evaporatively,” says study co-author Blair Wolf, a University of New Mexico ornithologist who’s studied birds in southwestern U.S. deserts since the mid-1990s. “Sweating removes huge amounts of heat.”
Birds and small desert mammals can’t sweat. Instead, they pant. By breathing quickly, they push air over moist surfaces of their lungs, throats, and mouths. When the water evaporates, it takes heat with it. Some birds can pant in overdrive—a mechanism known as gular fluttering, wherein they vibrate moist throat membranes to evaporate more water and expel more heat. A nightjar, for example, has “a mouth that’s like a big funnel for catching moths and other flying insects,” Wolf says. “They oscillate it really rapidly and can cool their bodies really effectively.” Small mammals have one other trick: They lick their fur. When the saliva evaporates, heat exits.
Riddell built complex computer models, incorporating his museum measurements with environmental and climate data, to generate what he calls “physiological landscapes.” The analysis showed that Mojave birds need 3.3 times as much water to stay cool compared to mammals. What’s more, climate change increased the birds’ cooling costs by 58.5 percent. Meanwhile, the mammals’ cooling costs increased by only 17.4 percent. To consume more water, birds have to find or catch more food, which itself takes more energy and requires more time in the sun. Riddell calculated that American Kestrels, the species that experienced the greatest decline, need to catch 15 to 20 more grasshoppers every day to compensate for their lost water. (Unfortunately water-rich fruits are in short supply in the Mojave, Wolf says; in the nearby Sonoran Desert, saguaro cactus allow diverse bird populations to survive droughts.)
Mammals have another important advantage: behavior. Desert mammals can burrow underground and fully escape the sun during the heat of midday. But birds don’t burrow. They can find some shade, but the time they spend hiding from the sun is time they’re not spending getting the food and water they need or reproducing. They can either stick it out in the deteriorating conditions or leave the desert to find better habitat elsewhere.
“Could birds adapt to how hot it’s getting? Probably not that quickly. We’re starting to push the limits of physiology here,” Riddell says. “There are certain relationships that organisms just can’t escape.”
The new research paints a bleak picture of the future of desert birds. Researchers across the world, including in Australia and South Africa, are finding similar patterns: Birds need increasing amounts of water, and when they can’t get it their populations dwindle. This effect is likely to only increase as climate change advances. Wolf suspects that at warming of 4 degrees Celsius, which he anticipates by the end of the century without significant efforts to reduce carbon emissions, many desert birds will blink out. “They’ll have shorter foraging windows in the morning and evening, and greater periods where they have to fast and evaporate water to stay cool,” he says. “They’ll be losing water all day and barely foraging. How much water can they lose before they die?”
The findings also hold a warning for humans. While we are much better than birds at evaporating water and staying cool, we have our limits. Evaporative cooling only works if the air is dry enough to take up water. Under high humidity, sweat drips off our bodies instead of evaporating, which means it’s not cooling us down. Some places are projected to get hotter and more humid with global warming, which means our innate air-conditioning systems might fail, similar to what is happening with the Mojave’s birds. “We’re looking at places in the [U.S.] south that you will not be able to work outdoors under future climates,” Wolf says. “There are areas [of the world] that will be potentially uninhabitable for some humans during the year.”
The new study’s findings are depressing and perhaps hard to accept, but they also demand our attention. “If you are a birder, the desert is going to look a lot less interesting in the coming decades,” Riddell says. “It’s important that everyone understand the gravity of the evidence in this case of the Mojave Desert,” he continues. “A 42 percent decline in these species is huge. Almost half are gone.”
For Andrea Jones, director of bird conservation with Audubon California, the results of the new study compel her to take action. “It’s more evidence that at a policy level we need to slow down the rate of climate change, and then at the local level land managers need to try and anchor some of these places where birds could still survive,” she says. Jones points to the removal of water-sucking invasive species like tamarisk and the restoration of desert springs and stream habitats to create shade and provide water for desert birds.
The same goes for ensuring human health and safety. Both large-scale policies to reduce carbon emissions across our power system and local investments in renewable energy, transportation infrastructure, and landscape restoration are needed to reduce the climate risks to the most vulnerable people, giving everyone a fair shot at staying cool long into the future.