New data show the populations of some of America's well-known birds in a tailspin, thanks to the one-two punch of habitat fragmentation and, increasingly, global warming.
From the heartland's whippoorwills and meadowlarks to the Northern bobwhite and common terns of the nation's coasts, 20 common bird species tracked by the National Audubon Society have seen their numbers fall 54 percent overall since 1967, with some down about 80 percent, the group reported Thursday.
Most of the trouble lies with loss of bird habitat, and has for decades, due to expanding agriculture and suburban development. The Rufous hummingbird's population has fallen 58 percent due to logging and development in its Pacific Northwest breeding range – and in its winter range in Mexico. The same thing has happened to whipporwills, whose numbers are down 57 percent due to loss of their forest habitat. At the same time, scientists say changes in migration patterns due to global warming are emerging, too.
"Habitat loss is still the major concern," says Greg Butcher, Audubon's bird conservation director in an interview. "But we're also seeing increasing impact from large-scale problems like global warming."
Thursday's study updates and expands earlier efforts: It adds to the annual Breeding Bird Survey, which is done by the US Geological Survey, some 40 years of data gathered by thousands of volunteers from the Audubon Society's Christmas Bird Count program. Together, these data make the new study one of the most comprehensive looks at bird-population trends in North America.
Crows, bluebirds, mourning doves, and robins are wintering farther north and their populations are still robust, Dr. Butcher says. But other species that rely on cold climates, like the snow bunting and greater scaup, which breed in Alaskan and Canadian tundra, are showing signs of trouble. As the tundra warms, it cedes to shrubs unsuitable for the birds to breed, Butcher says. Scaup numbers are down 75 percent and bunting 64 percent, in part because these species can't go farther north to breed.
Although populations in some cases dip as low as 500,000 globally, none of the "common" bird species cited is in danger of extinction, Butcher notes. But their struggle mirrors a larger global trend among endangered migratory songbirds and tropical species.
At the current rate, global warming and destroyed bird habitat could lead to the significant decline or extinction of at least 400 of the world's 8,750 bird species, reported another new study, this one focused on climate-change impact. Under certain scenarios, 750 to 1,800 bird species could be imperiled by the year 2100, the study released earlier this month reported.
Most of the threatened populations are in the tropics and are in danger due to slashing of their rain-forest habitat – but global warming is a clearly a looming threat, says Walter Jetz, an assistant professor of biological sciences at the University of California at San Diego and lead author of the study.
"It's true some birds are responding already to climate change by moving farther north and leaving later in fall and coming back earlier," he says. "But there's a limit to that. At some point, there's going to be real trouble because of the impact and distortions on migration routes that have evolved over millions of years."