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Mystery Of The Shocking Declines In California’s Birds

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Climate change is not the only danger to California’s birds. Throughout the course of the twentieth century, urban expansion and agricultural growth transformed the state’s environment, pushing many native species to adapt to new and unfamiliar settings.

Biologists from the University of California, Berkeley, used current and historical bird surveys in new research to show how land use change has accentuated — and in some instances alleviated — the effects of climate change on bird populations in Los Angeles and the Central Valley.

The study found that over the past 100 years, more than one-third of the bird species in the L.A. area have gone extinct because of urbanization and warmer, drier weather. In the Central Valley, where agriculture is growing and the weather is getting warmer and a little bit wetter, the effects on biodiversity are more complicated.

“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,” adds study senior author Steven 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.”

The study, which was published this week in the journal Science Advances, presents the most recent findings from the Grinnell Resurvey Project at UC Berkeley, an initiative to revisit and document birds and small mammals at locations that professor Joseph Grinnell of UC Berkeley first surveyed more than a century ago.

In the present study, birds were resurveyed at 71 locations in the Central Valley and Los Angeles. They later examined how changes in the climate and landscape may have influenced changes in bird populations using their results combined with recent and old data on land use, average temperature, and rainfall.

In Los Angeles, they found that 40% of bird species were found at fewer sites than they were 100 years ago, while only 10% were found at more sites. In contrast, the number of species in the Central Valley that suffered a fall (23%) was just more than the number that had an increase (16%). The effects of climate and land use change on individual bird species were often buffered by the fact that they reacted in opposite ways to the same changes.

The loss of bird species in L.A. over the last 100 years is similar to the shocking loss of bird communities in national parks in the Mojave Desert over the last 100 years, which was linked to climate change and heat stress on birds.

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

In the late 1890s in Pasadena, California, Grinnell began documenting birds. Later, while serving as the first director of the MVZ and a professor of zoology at UC Berkeley, he refined his meticulous technique to surveying.

“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,” Beissinger adds. “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.”

Grinnell’s thorough field notes enabled Beissinger and his colleagues to create a historical baseline of California’s bird life at the start of the twentieth century. The notes are so precise that the researchers can rebuild the birds they see every day and account for how new technology, such as improved binoculars and field guides, have made it simpler for modern biologists to discover birds. The team was able to establish direct comparisons between current and previous bird surveys thanks to this technique.

In order to assess how the terrain at each study site had changed over the 20th century, the researchers examined historical maps of urban growth and agricultural. This allowed them to separate apart the different and sometimes conflicting influences of land use change and climate change. At each location, they also gathered data on historical average temperatures and rainfall.

They discovered that in Los Angeles, species like the American crow and Anna’s hummingbird were able to adapt to both hotter and drier circumstances as well as urban expansion, experiencing what the researchers refer to as a population “windfall.” Some species, such as the western meadowlark and the lark sparrow, suffered a “double whammy” as a result of these changes.

The great egret, the house wren, the black phoebe, and the blue-gray gnatcatcher are among the species that have had mixed effects.

“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,” explains Beissinger. “Sometimes, species might also be pushed and pulled in different directions from the climate and land use changes.”

Windfalls, double whammies, and mixed effects were all experienced by bird species in the Central Valley, although the percentage of species that received windfalls was substantially greater than in L.A. and virtually offset the proportion that experienced double whammies.

Several species have managed to survive the changes brought about by agriculture, and others have even colonized new areas and multiplied as a result of those changes. Yet, they often belong to more broad and widespread species, and the more delicate species are the ones that began to go extinct when the natural grasslands were replaced by agriculture, according to Beissinger. 

“In the urban areas, there are just fewer species that are able to find what they need and avoid the city hazards.”

Source: 10.1126/sciadv.abn0250

Image Credit: Getty

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