“I don’t think there’s anybody today that could hold a candle to what those guys could do, both physically and in terms of deep thinking about what they were seeing,” Patton says. “We’re too keyed to our mechanical, digital devices to actually look at things.”
While Grinnell collected and surveyed specimens by shotgun and deadly snap traps, the Pattons set 200 live traps each night, recording their catch over four or five days. They preserved a few specimens for the museum, spending eight weeks every spring and six weeks every fall from 2015 through 2018 in Death Valley.
The results were unexpected, Beissinger says. “We were very surprised that what we saw was resilience for the small mammals,” he adds. “When we looked at the proportion of sites that a small mammal occupied a century ago, when Grinnell and his colleagues were out [surveying] and the proportion we have now, it was almost a straight line.”
Patton, who has studied small mammals for more than 50 years, says they are buffered by their nocturnal nature, their burrows, and their ability to meet water needs by metabolizing seeds. Birds, meanwhile, forage in the heat of the day and often require open water sources, like springs, pools, and surface waters. “Many of the birds require exogenous water to survive,” he says. “But most of the small mammals out there are manufacturing their own water [through seeds].”
Beissinger puts it in simple terms: Birds have more exposure to heat and are more sensitive to the effects of climate change.
To understand those differences, they brought in Eric Riddell, an assistant professor of ecology, evolution, and organismal biology at Iowa State University. Riddell had been a postdoctoral researcher at Berkeley who built computer models to calculate the cooling needs of 49 desert birds. While Patton and his wife were camping in the desert, Riddell camped out at the museum, spending six months over two years beginning in 2017 taking measurements of bird specimens, determining their rough dimensions, the length and density of their feathers, and even how much sunlight bounces off of them or is likely to pass through their plumage to their skin. From models created using those measurements, he was able to estimate the amount of extra water needed for evaporative cooling by each bird species today compared to 100 years ago. The species that declined from Grinnell’s time were the ones that had the most difficulty keeping cool, notably larger birds, especially those like the violet-green swallow and the white-throated swift that get most of their water from insects.
For small mammals, he returned in 2019 to do the same, cataloging body size and fur density for another six months. The models looked at how their bodies absorbed or reflected heat, including direct sunlight, reflected sunlight, and radiant heat from the ground. A rodent with fluffy fur might transfer that heat slowly while one with short fur, like a ground squirrel, might transfer it quickly.
His program simulating the effects of climate change—increased temperatures and decreased precipitation—consisted of more than 1,000 lines of code. Riddell used UC Berkeley’s supercomputer: 240 linked computers, running for 18 hours to calculate 1.2 billion hourly simulations. Translated, that means the model calculated how much heat each species of mammal gained or lost every hour of every day over the last 100 years in the Mojave Desert.
The key to the different outcomes for birds and mammals proved to be water consumption. Riddell found that birds required almost three times as much water as small mammals to cool themselves. “In the desert, water is very limiting, and there isn’t much of it. And you need that water to cool off,” he says. “In the last century, birds experienced this really massive increase in the amount of water that they needed just to stay cool, just to function, and small mammals haven’t experienced that change.”