Many bird species commonly found in California have suffered steep population declines, as much as 96%, part of a nationwide trend that is due in large part to diminished habitat, according to a study that for the first time combines 40 years of data.
The study, combining the National Audubon Society's Christmas season bird counts with summertime surveys by the U.S. Geological Survey, documented declines of 75% to 96% in several California species, including the northern pintail, horned lark and loggerhead shrike.
Greg Butcher, Audubon bird conservation director and analysis leader, said of the birds surveyed nationally, "about half are in decline and of these half are in significant decline." Overall, Butcher said, his organization is concerned about decreasing numbers of 200 to 300 kinds of birds. Because they have been so numerous, the birds that were surveyed often don't get the attention that small, endangered populations such as California condors have received. However, Gary Langham, Audubon California's director of bird conservation, said that although many bird populations may number in the millions, the large reductions that were documented are statistically significant.
Daniel Cooper, a Pasadena-based conservation biologist and the author of "Important Bird Areas of California," said of the surveys, "They cover a period of history that has seen just enormous changes in the landscape in the U.S., and we don't really have comparably long data sets like that."
Kimball Garrett, collection manager for ornithology at the Natural History Museum of Los Angeles County, said that while the surveys must be interpreted cautiously, they are in line with other studies that show steep declines.
"Some trends are just undeniable," he said.
The evening grosbeak fared the worst in Audubon's California survey, with a 96% decline statewide. Nationally, grosbeak numbers fell from 17 million 40 years ago to 3.8 million today, according to both surveys. The black, yellow and white birds, about the size of a robin, live in California mountains where their habitat has been reduced by logging and development.
The causes of the declines differ slightly among species, but Audubon scientists say that habitat loss is the most common factor, and they attribute the losses largely to the expansion of urban areas and the conversion of grasslands to agriculture, said Graham Chisholm, Audubon California's conservation director.
"We have done a reasonably good job of protecting some habitats, but we've already destroyed so much that remaining parts are very critical," Chisholm said. "People really need to support the remaining areas we have."
Important habitats include oak woodlands in the Sierra foothills and Antelope Valley grasslands -- two areas experiencing widespread development. Chisholm said the Antelope Valley is the last stronghold of the tricolored blackbird. He said oak woodlands are home to species found only in California such as Nuttall's woodpecker and the yellow-billed magpie.
Keith Pardieck, national director of the USGS bird count, said his agency has noticed a significant decline in grassland birds in many parts of the country over the last decade.
"The bigger question of the long run," Chisholm said, "is how does global climate change have an impact on these habitats?"
Langham said the study points to the likelihood of similar declines in plants and other animals. "Birds are much more easily surveyed than mammals because they are out in the daytime, they are visual, they sing songs, they are easy on the senses," he said. "The declines across so many [bird] species in different habitats underscores the breadth of the problems facing birds and other wildlife."
The annual surveys by Audubon and the USGS employ thousands of volunteers. In the USGS census, the volunteers drive the same 24.5-mile route every year and stop at half-mile intervals to look for birds for three minutes, recording what they hear and see. In the Christmas bird counts, Audubon volunteers spend an entire day counting birds in a circular area 15 miles in diameter. Both surveys record all birds they encounter, not just species.
The USGS census has taken place since 1967, and the Christmas bird counts for more than 100 years.
In addition to participating in bird surveys, Langham said, people can help local Audubon chapters with habitat restoration and by adding native trees and plants to backyards. However, he said the national scope of the declines and the fact that many birds migrate from one part of the country to another show that local conservation must be complemented by changes in policy to protect remaining habitat.
There's still time to arrest the declines, Chisholm said.
"These birds are down but not out."