INTRODUCTIONS OF NON-NATIVE SPECIES
While European Starlings, Rock Pigeons, and House Sparrows may be the most visible of the non-native Sierra birds, no species has had a larger impact on native breeding birds than Brown-headed Cowbirds. Originally native to the Great Plains, where they followed herds of bison and pronghorns, cowbirds were unrecorded in California before 1870, although some may have been present on the East Side of the Sierra decades before that. By the 1930s, however, they were common and widespread in the Central Valley and Sierra foothills as they spread northward from Mexico and Arizona, taking advantage of the livestock grazing that accompanied human incursions into the Sierra.
Since then, they have moved steadily into higher and higher elevations. Sierra birds have not adapted to this brood parasite that lays its eggs in the nests of other birds. Effects remain relatively localized at higher elevations but are widespread in much of the low- and middle-elevation habitats that support more abundant cowbird populations. The populations and ranges of susceptible host species such as Willow Flycatchers and Yellow Warblers shrank dramatically because these species were unable to successfully fledge their own young while simultaneously raising a cowbird chick. The introduction of aggressive cavity nesters such as European Starlings and House Sparrows has likely impacted some native cavity-nesting birds. However, the fact that starlings and House Sparrows are mainly associated with urbanized areas has limited their impacts in the Sierra. It is hard to assess whether or not more recent intentional introductions (e.g., Wild Turkeys and White-tailed Ptarmigans) or rapid range expansions (e.g., Great-tailed Grackles and Eurasian Collared-Doves) will affect native Sierra birds in the future.
POLLUTION, PESTICIDES, AND OTHER ENVIRONMENTAL CONTAMINANTS
There may be no more compelling conservation success story than the one surrounding the banning of DDT in the United States in the late 1970s. This pesticide caused eggshell thinning, which had devastating impacts on populations of many bird species (see the “Family and Species Accounts” section of the book for the accounts of Bald Eagle and Peregrine Falcon as examples). Once the link was proven and the compound banned, the affected species recovered to the point that some have been removed from the Endangered Species List.
Continued diligence and careful research on pesticides is needed, however, as hundreds of new chemicals enter the environment every year and there is generally little testing for impacts on wildlife. Some have suggested links between environmental contaminants and recent widespread declines in some species such as Loggerhead Shrikes and American Kestrels. However, no solid evidence has emerged to confirm a link or identify a specific compound. Human development along the shores of Lake Tahoe has contaminated the once-pristine waters there, and air pollution from urban traffic has severely affected air quality in the foothills east of Sacramento, which has been shown to stress pine trees and other native vegetation and could be leading to subtle habitat changes. Direct impacts on birds have yet to be shown in either case, but degradation of water or air quality is likely to have widespread consequences across the entire natural community.
LAND USE CHANGES
Although many recent trends in human attitudes and practices are cause for optimism, land use changes in recent decades pose major threats to some Sierra habitats. The most serious threats are to the grassland, savanna, and chaparral habitats of the West Side foothills. Nearly all these lands are in private ownership and could be developed in the future. In recent decades residential and rural residential development has impacted the foothills more than any other part of the Sierra. In addition, thousands of acres have been converted from relatively wildlife-friendly cattle ranching to orchards and vineyards. As compared to the huge vineyards in the Central Valley that consume entire landscapes, much of the vineyard land in the Sierra occurs in smaller patches within a matrix of natural habitats. However, large contiguous areas of habitat are fragmented into smaller and smaller parcels, and efforts to protect homes and crops from the threat of fire (a natural, and critical, component of these ecosystems) impact all the land adjacent to these new land uses.
SIERRA BIRDS IN A CHANGING CLIMATE
Predicting the impacts of global climate change on the Sierra and its birds is particularly challenging. The nature of California’s climate is inherently complex due to the effects of long-term and short-term weather pattern cycles in the Pacific Ocean and the highly varied topography of the state. While California has seen some warming during the past century, the changes are less dramatic than in many other regions of the United States. Most climate models predict a warmer Sierra climate, possibly including more precipitation, but with more of that precipitation falling as rain instead of snow. These models also predict that by the second half of the 21st century, temperatures in the Sierra foothills could increase by 3.5° to 7.5° F and the frequency of extremely hot days (greater than 100° F) could almost double. Data from recent studies from areas of the Sierra first surveyed a century ago suggest that birds are indeed gradually moving (generally upslope) to remain within relatively similar climate zones.
As the climate changes, bird species are expected to shift their distributions independently, in some cases resulting in combinations of co-occurring species that have not been seen before. Species using the highest altitudes for breeding (e.g., American Pipits, Gray-crowned Rosy-Finches) may be unable to find suitable habitat in the future. Changes in winter snow-pack and spring temperatures could affect downstream riparian systems by reducing the amount of water and changing the seasonal timing of peak flows. These changes could alter the streamside vegetation and effect birds using those habitats.
HOPE FOR THE FUTURE
A historic 1994 decision by the State Water Resources Control Board reversed a long-term decline of the Mono Lake ecosystem caused by more than fifty years of water diversions from its principal tributary streams. This decision restored the stream flows and will eventually increase the surface elevation of Mono Lake to an average of 6,392 feet, which should ensure that this critical ecosystem will provide suitable habitat for myriad water birds in the future. Ongoing restoration efforts at Owens Lake and the upper Owens River, also degraded by historic water diversions, are also important steps toward restoring essential bird habitats in the eastern Sierra.
Two comprehensive reviews of the conservation status of the Sierra, the Sierra Nevada Ecosystem Project (SNEP) (California Resource Agencies 1996) and the Sierra Nevada Framework analysis (U.S. Forest Service 2001) came to the identical conclusions that this region faces urgent threats. In 2004 the SNEP report helped catalyze the formation of a new state agency, the Sierra Nevada Conservancy, which has the mission of initiating and supporting efforts that “improve the environmental, economic and social well-being of the Sierra Nevada Region, its communities and the citizens of California.” This organization is tangible evidence of the high value that Californians place on protection of the Sierra and recognition that land use policies and management must focus on maintaining healthy ecosystems that provide high-quality water, spectacular scenery, and important wildlife habitat for all of California.
SUMMARY
Despite the challenges described here, we have good reason to be hopeful about bird conservation in the Sierra. Never before have so many people cared so deeply about nature in general, and birds in particular, and been willing to work to preserve natural habitats. Regulatory protections in place today would have been unthinkable even 50 years ago, and the new Sierra Nevada Conservancy provides a forum for collaborative, science-based approaches to managing Sierra bird and other wildlife populations. Most of the Sierra above the foothills is in public ownership, and many of the largest private landowners have strong commitments to good land stewardship.
Birds demonstrate to us again and again their astounding capacity to adapt. Indeed, the data summarized in the chapter “Recent Trends in Sierra Bird Populations and Ranges” suggest that more species in the Sierra are increasing than decreasing. More than almost any other organisms, birds are highly mobile, and many species can find and colonize new areas of habitat quickly. As human populations continue to increase and competition for critical resources like water and open space becomes more intense, we must combine our capacity to invent with our unique capacity to appreciate