Extreme heat triggers mass die-offs and stress for wildlife in the West

As the Pacific Northwest baked in 115 degree heat last month, fuzzy baby hawks sat sweltering in their nests, 50 feet off the ground. Unable to fly, the young raptors dealt with the heat in the only way they could: One by one, they threw themselves out. 

Nearly 50 baby Cooper’s and Swainson’s hawks were rescued from the ground beneath towering pines in Washington and Oregon and brought to Blue Mountain Wildlife, a rehabilitation organization in Pendleton, Oregon, which specializes in treating birds of prey. More still were brought to Portland Audubon and other rehabilitation facilities throughout the Pacific Northwest.

The historic heat wave coincided with nesting season, says Lynn Tompkins, director of Blue Mountain Wildlife. If the young birds had been able to fly, they could have sought reprieve in a cooler spot. If they’d had feathers, they’d have been able to regulate their body temperatures. “But these guys were just downy babies,” she says, “and there was nothing to do but bail out.” 

The hawk nosedives are one dramatic example of the many ways wild animals have been affected by extreme heat in the West. Marine life, including mussels and sea stars, have died en masse from exposure to unusually hot air. One estimate puts the death toll at more than a billion. Other effects aren’t yet clear. In some cases, human development prevents animals from being able to flee to cooler areas. Other animals are likely to take new risks by venturing places they normally wouldn’t, in search of shade or water. 

As extreme heat events become more frequent and median temperatures rise, experts are concerned about animals’ ability to survive and adapt.

Stressor on top of stressor

To understand how wildlife is affected by extreme heat, says Mažeika Sullivan, a professor at Ohio State University's School of Environment and Natural Resources, it’s important to keep in mind that heat waves are just one of the challenges wildlife face. 

“It’s about multiple stressors,” he says. Extreme heat events are compounded by drought, rising temperatures, bigger and more intense wildfires, and increasingly fragmented habitats. (This is how animals cope with wildfires.)

Animals “only have so many coping mechanisms,” he says. “And asking them to cope with so many environmental stressors that are happening over long time scales and then heat waves for multiple days—it’s impossible to talk about one without talking about it in context.”

Salmon, for example, are migratory fish, moving from the ocean up rivers to spawn. They’re already stressed by the network of dams they need to get over or around, Sullivan says. 

But during a heat wave, they’re hit with “a double whammy,” says Jonathon Stillman, adjunct biology professor at University of California, Berkeley, who researches how environmental changes affect marine life. Salmon face both increased water temperature and decreased oxygen, he says, because warmer water contains less of it. “It’s like if you had to run a marathon while wearing a plastic bag over your head, while it’s 10 degrees warmer outside,” Stillman says. “You’d die too.”

In California’s Sacramento River, it’s a “crisis situation” for young, endangered Chinook salmon  in particular, says Jordan Traverso, spokesperson at the California Department of Fish and Wildlife. The fish cannot survive beyond their egg stage in waters heated by extended temperatures that exceed 100 degrees. In an effort to save as many as possible, the department has transported more than 16 million juveniles from four hatcheries in the Central Valley to seaside net pens in San Pablo Bay and San Francisco Bay.

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Other aquatic species that live in shallow waters have also been hit hard, according to researchers from the University of British Columbia, who estimate that hundreds of millions of mussels likely died in the heat waves, essentially steaming in their shells, as well as sea stars, barnacles, hermit crabs, and other shoreline-dwelling organisms. These creatures are used to being exposed to the air when the tide is low, but they aren’t evolved to cope with such extreme air temperatures as the West has been seeing, Sullivan says. (Read about thousands of flying foxes that died in Australia’s extreme heat.)

Who survives?

Environmental stressors kill animals all the time. That’s just part of the evolutionary process, says Stillman: the sometimes-overlooked inverse of the survival and reproduction of animals hardy enough to withstand those stressors. The problem is if there isn’t enough genetic variation to keep up with the rapid pace of climate change, which includes more frequent and widespread heat waves. Animals now have only a few decades to adapt to changes that historically occurred over millions of years. The hope is that every species has enough extreme-heat-tolerant individuals that they can pass on their genes to offspring. “The alternative is pretty bleak,” Stillman says. “The alternative is that there are no individuals that can survive.”

“We might expect all [species] will be affected to some degree,” Sullivan says, but those effects will be on a spectrum, depending on a species’ exposure to heat, their physiology, and their vulnerability.

Salamanders, for example, are vulnerable because they can’t handle desiccation—a total absence of moisture from their skin. Birds that are diurnal—active during the day—may fare worse than nocturnal birds. Animals that spend their lives above-ground may have a harder time than burrowing animals, Sullivan says. 

An animal’s behavior is also a factor in their ability to handle heat, he says. Birds spread their wings and pant to cool off, which uses water. But, for birds in the Mojave Desert at least, the more water a species requires, the more likely its numbers were to plummet, according to a 2019 study of bird populations there.

No escape

Many wild animals rely on having large expanses of nature through which to move. “But human development has cut [those paths] off,” Stillman says. Roads, farmland, and other infrastructure impedes their ability to move to cooler areas or forces them to expend more energy to try to find them. Habitat fragmentation is also an issue for aquatic wildlife, Sullivan says. Dams and waterway fragmentations have reduced “refugia”—deep, shaded pools of water where it’s cooler at the bottom.

Sullivan also expects to see increased human-wildlife interaction amid extreme heat waves and drought. “Animals will most likely be taking risks they wouldn’t normally take to seek water,” he says, such as passing through urban areas and crossing roads. In late June, a viral video showed a mother bear and her three cubs wandering onto a busy Lake Tahoe beach to swim as onlookers filmed from nearby.

In much of the Pacific Northwest, temperatures have dropped again—for now. (But massive wildfires are raging in southern Oregon—the largest so far this year in the U.S., having already burned through 530 square miles of forest and grassland.) Many of the baby hawks brought to Blue Mountain Wildlife have made a full recovery. The Cooper’s hawks have all been released, and the Swainson’s hawks are almost ready. 

Still, director Tompkins says, “it’s kind of scary. The more hopeful side of my brain says it was the perfect storm [of freakish extreme heat],” that caused the hawks to ditch their nests. “But the logical part of my brain says this is what the future holds.”

“Drought affects us all,” says Traverso of the California Department of Fish and Wildlife, which is closely monitoring impacts of the heat and drought on wildlife populations. She says her team is often reminded of a quote by poet W.H. Auden: “Thousands have lived without love, not one without water.”