Collapse of desert bird populations likely due to heat stress from climate change

The team that last year documented a collapse of bird communities in Mojave Desert over the last century -- 29 percent of the 135 bird species that were present 100 years ago are less common and less widespread today -- has now identified a likely cause: heat stress associated with climate change.

The researchers' latest findings, part of UC Berkeley's Grinnell Resurvey Project, come from comparing levels of species declines to computer simulations of how "virtual birds" must deal with heat on an average hot day in Death Valley, which can be in the 30s Celsius -- 90s Fahrenheit -- with low humidity. These temperatures are, on average, 2 C (3.6 F) hotter than 100 years ago. The birds that the model predicted would require the most extra water today, compared to a 100 years ago, were the species that had declined the most in the Mojave Desert over the past century. The desert straddles the border between California and Nevada.

The most threatened turn out to be larger birds, and those that have an insect or animal diet.

"We often think that climate change may cause a mass mortality event in the future, but this study tells us that the change in climate that has already occurred is too hot, and in certain areas, animals can't tolerate the warming and drying that has already occurred," said lead author Eric Riddell, a UC Berkeley postdoctoral scholar.

The virtual bird simulation was unique in allowing the researchers to identify the impact of a daily physiological stressor in the desert -- heat -- as birds leave the shade to forage for food or find mates. Other aspects of a changing environment, such as changing food sources and fire, only add to the heat stress.

"This is one of the first studies that directly ties the increase in physiological demands from a warmer and drier climate to the changes that are taking place in biodiversity," said senior author Steven Beissinger, a UC Berkeley professor of environmental science, policy and management and a researcher at the Museum of Vertebrate Zoology at Berkeley. "Most previous studies have not found a direct physiological connection between climate change and biodiversity change, which is usually mediated through changes in the food web or competing species. Our study points to a direct effect of climate change via increased water demands for evaporative cooling to maintain body temperature in the comfort zone."

According to Beissinger, the team's conclusions about these California and Nevada desert birds may apply to species in other regions of the world.

"Warmer, drier conditions are expected to spread with climate change, so we are probably looking at an increase, for birds in this kind of evaporative water demand, in a lot of places," he said. "Some of these effects might not just be limited to the desert. Does it matter to an insectivore that doesn't drink water if it is in the hot desert or in your hot hometown, say Atlanta? As the climate warms, I am not so sure."

Beissinger, Riddell and their colleagues detailed their findings in a paper to be published this week in the journal Proceedings of the National Academy of Sciences.

The Grinnell Resurvey Project, a 15-year effort to revisit and record wildlife at sites around California visited by UC Berkeley biologist Joseph Grinnell and his colleagues at the Museum of Vertebrate Zoology between 1904 and 1940. The comparison of the state's mammal and bird life over more than a century has revealed the changes wrought by climate change to date and provided insight into what may happen in coming decades as global warming continues.

Panting and gular flutter

Like humans, birds regulate their internal temperatures to keep within a comfortable range. But they do not sweat. Like dogs, they pant, but can also vibrate their throat muscles in what's called a gular flutter. The resulting increase in air flow and evaporating water cools them off.

The hotter they get, however, the more water they must exhale to lower their body temperature. The team calculated that larger birds, like the mourning dove, require 10% to 30% more water today to keep cool because of the 2 C increase in Mojave Desert temperatures over the last 100 years.

According to the UC Berkeley analysis, birds that eat insects or other animals are more threatened by changes in evaporative water loss because they typically get all of their water from the moisture in their food. They seldom, if ever, drink from surface water sources. A 30% increase in water requirement could mean that larger birds have to catch an extra 60 to 70 bugs per day to survive the increased heat. If those bugs are even around, the birds still have to expend extra energy and time to find them.

The team's field survey, led by postdoctoral researcher Kelly Iknayan and published last year, confirms this prediction: The American kestrel, prairie falcon and turkey vulture, all large and carnivorous, have declined, as have large insect-eaters like the white-throated swift, violet-green swallow, olive-sided flycatcher, Western meadowlark and Western bluebird.

Smaller birds that eat seeds or are omnivores are less threatened, according to the model. In fact, the earlier study showed that small insectivores in the Mojave Desert -- the blue-gray gnatcatcher, ruby-crowned kinglet, mountain chickadee, black-tailed gnatcatcher, black-throated sparrow, verdin and canyon wren -- have suffered less over the past century.

Vegetarian birds, such as seedeaters, face a different problem. Because they can drink from surface water sources -- springs and pools in desert oases, they can supplement the water they get from their food. But that's only if water is around. Even protected areas of the Mojave Desert -- Death Valley, Joshua Tree National Park and the Mojave National Preserve, where the bird surveys were conducted -- are getting drier from climate change and because of groundwater pumping by nearby cities and agricultural areas. As a result, the seedeaters are also at risk of heat-related death.

The team's survey of birds in these parks confirmed that most seedeaters in areas with open sources of water have been impacted less by climate change over the last century.

"For plant-based eaters, it is more binary: whether or not a species survived at a site over the past 100 years had more to do with the presence or absence of surface water. If you could drink, you were better off than if you couldn't drink," Riddell said "For insect eaters, it is more dependent on the magnitude of cooling, determined by body size and feather absorbance. The greater its water requirements, the more a species declined. So, there are different ways in which climate change is manifesting itself for different members of the bird community."

The researchers noted that some birds are adapting to the hotter temperatures by moving northward or up mountain slopes to find cooler habitats, while others are shifting their active nesting periods earlier in the year to avoid hotter summer temperatures. Some species are even becoming smaller, reducing their water needs. But birds can reduce their size only so much.

"We explored that possibility: how small would birds have to get to maintain the same levels of stress," Riddell said. "It was something ridiculous, like 35 to 50 percent smaller. That's not gonna happen, it is way too small."

Virtual birds

Riddell, who has a background in physics, modeled the heat balance of 50 different species of desert birds on a computer, obtaining physical data on each from specimens in the Museum of Vertebrate Zoology. These data included size, as well as reflectance of feathers, length of feathers and depth of feathers on the back and belly. He validated the model by comparing its predictions to data on heat balance in captive birds and also to experiments on physical bird models by researchers, including co-author Blair Wolf of the University of New Mexico in Albuquerque.

He then ran the model, varying conditions for each bird species -- from staying in the sun all day to remaining in the shade all day -- at various temperatures. Death Valley holds the record high temperature on Earth: 134 F in 1913.

From these virtual experiments, Riddell and his colleagues were able to obtain good estimates of the amount of extra water needed for evaporative cooling by each bird species today compared to 100 years ago, and what these birds will need in the future, under different climate change scenarios.

Not surprisingly, larger birds turned out to require a higher amount of water today, and they represent many of the species most impacted since the early 20th century. Riddell predicts that, in the worst case scenario of climate change, larger birds like the mourning dove may require nearly twice their typical intake of water by the end of the century to remain cool enough to survive higher temperatures in the desert.

"People have been focusing on lethal thresholds for birds, but our metric was death by a thousand cuts: recurrent cooling costs that birds are going to have to deal with every day," Riddell said. "According to Kelly's work, we have seen a 50 percent reduction in species diversity at sites in the desert visited a century ago by Grinnell. This isn't really a question of when will it happen in the future, but understanding what we have already done, what has already happened."

Barry Sinervo of UC Santa Cruz was also a co-author of the paper. The work was funded by the National Science Foundation (1457742).