Among California’s bird species, there are winners and losers

Joseph Grinnell, born in 1877, was an American field biologist and zoologist who served as the first director of the Museum of Vertebrate Zoology and a professor at the University of California, Berkeley. One of Grinnell’s goals for the museum was to build a specimen collection of the fauna of California, a project he began in 1908, with his first field expedition to the Colorado Desert. Over time, he surveyed the birds, mammals and other vertebrates of California, right up until he died, in 1939.

Grinnell developed a systematic and detailed protocol for recording field observations involving not only the specimens he collected, but also written records kept in notebooks and journals. He kept accounts of species’ behaviors, made maps, took photographs of collecting sites and documented weather conditions, vegetation types, vocalizations and other evidence of animal presence. 

These very detailed records have enabled scientists at UC Berkeley to conduct a subsequent resurvey of most of the original sites, and to compare the numbers of animals with Grinnell’s original findings in order to understand they ways in which animal populations have changed. 

In a report published in the journal Science Advances, the biologists present the latest results from the Grinnell Resurvey Project, particularly those relating to the numbers of birds and small mammals at the sites first surveyed by Grinnell, more than a century ago. The researchers were able to resurvey the birds at 71 of the original sites in L.A. and the Central Valley, and to use the comparisons to identify how shifts in climate and land use may have contributed to changes in the bird populations.

The study revealed that urbanization and much hotter and drier conditions in L.A. have driven declines in more than one-third of bird species in the region over the past century. Meanwhile, agricultural development and a warmer and slightly wetter climate in the Central Valley have had more mixed impacts on biodiversity.

Study senior author Steven Beissinger is a professor of Environmental Science, Policy and Management at UC Berkeley and a researcher at the Museum of Vertebrate Zoology.

“It’s pretty common in studies of the impact of climate change on biodiversity to only model the effects of climate and not consider the effects of land use change,” said Professor Beissinger. “But we’re finding that the individual responses of different bird species to these threats are likely to promote unpredictable changes that complicate forecasts of extinction risk.”

In L.A., the experts found that 40 percent of bird species were present at fewer sites today than they were 100 years ago, while only 10 percent were present at more sites. Meanwhile, in the Central Valley, the proportion of species that experienced a decline (23 percent) only slightly outnumbered the proportion that increased (16 percent). In many cases, opposing responses to climate and land use change by bird species, where one threat caused a species to increase while another caused the same species to decline, moderated the impacts of each threat alone.

“The Central Valley had less change, in general – there were winners and losers,” Beissinger said. “Whereas in L.A., we saw mostly losers.”

To identify the roles of land use change and climate change in determining current bird numbers, the researchers analyzed historical maps of urban development and agriculture to establish how the landscape at each study site has been modified since Grinnell’s original survey. They also obtained data on historical average temperatures and rainfall at each site.

In L.A., they found that species such as Anna’s hummingbird and the American crow were able to tolerate the hotter and drier conditions, and the level of urban development, thus experiencing what the researchers call a population “windfall.” Other species, such as the western meadowlark and the lark sparrow, were negatively impacted by both categories of change, experiencing instead a “double whammy.”

Species that experienced mixed impacts include the black phoebe, the great egret, the house wren and the blue-gray gnatcatcher.

“Our findings really highlight the fact that we’ve got climate and land use change happening at the same time, creating happy conditions for some species, while other species are declining from the same changes,” Beissinger said. “Sometimes, species might also be pushed and pulled in different directions from the climate and land use changes.”

Bird species in the Central Valley also experienced a combination of windfalls, double whammies and mixed impacts, but the proportion of species that experienced windfalls was much higher in the Central Valley than in L.A. and nearly offset the proportion that experienced double whammies.

“There are some species that have been able to persist under the agricultural changes, and some that even colonized and increased because of those changes. But they tend to be species that are more common and widespread, and the more sensitive species are the ones that started disappearing when the natural grasslands were replaced by agriculture,” Beissinger explained. “In the urban areas, there are just fewer species that are able to find what they need and avoid the city hazards.”

The detailed field notes kept by Grinnell and his field biologists enabled Bessinger and his team to locate the original study sites, and to construct a historical baseline of California’s bird life at the turn of the 20th century. The notes are so detailed that the researchers were able to reconstruct the birds encountered on each day in the field, and to account for the ways in which new technologies, such as better binoculars and field guides, have made it easier for contemporary biologists to detect birds. 

“In those days, they didn’t have fancy binoculars. They didn’t have recordings of bird calls. So, they had to get in and learn the birds through the resources that were available. Oftentimes that was from specimens in museums. Sometimes that was through popular guides or handbooks,” said Bessinger. “Grinnell was ahead of his time in the way that he was taking field notes, and he was really draconian in also making all his students take those notes.”

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By Alison Bosman, Earth.com Staff Writer

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