Story Highlights • 20 common birds have lost more than half their populations in the past 40 years

• Birds in decline: Northern bobwhite, field sparrow and boreal chickadee

• Factors: Agriculture, habitat loss, pesticides, invasive species, global warming

• Health of a bird population often a harbinger of health of other wildlife, humans

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(CNN) -- Some of the most common birds seen and heard in American back yards are becoming a less frequent sight and sound in much of the United States, according to a study released by the National Audubon Society.

Twenty common birds -- including the northern bobwhite, the field sparrow and the boreal chickadee -- have lost more than half their populations in the past 40 years, according to the society's research.

"These populations are not yet on the endangered species list, but it is noteworthy, and we need to take steps to protect their habitat," said Carol Browner, Audubon chair and former Environmental Protection Agency administrator.

And like the proverbial canary in the coal mine, the health of a region's bird population is often a harbinger of the health of other wildlife and of human populations as well.

"The focus isn't really on what's happening to these 20 birds, but what's happening to their environment," said Greg Butcher, the society's conservation director.

The researchers say many factors play into the decline in bird numbers, including intensification of agriculture, other loss of habitat, pesticides, invasive species, and global warming.

Scott Weidensaul, an author and expert on bird migration, said he remembers waking up to whistles of bobwhite quail, and falling asleep listening to whippoorwills.

"Today you can't find a bobwhite in Pennsylvania, and hearing a whippoorwill is a red letter day," he said at an Audubon news conference Thursday morning.

The report shows the current northern bobwhite quail population is 5.5 million, down from 31 million in 1967. That's a decline of 82 percent in the past four decades. There are currently about 1.2 million whippoorwills now, down from 2.8 million 40 years ago, a 57 percent decline.

The Audubon Society created its list of "common birds in decline" by analyzing annual sighting data from the Audubon Society's century-old Christmas Bird Count program, and results of the annual Breeding Bird Survey conducted by the U.S. Geological Survey. The data will also be submitted for scientific peer review.

Changes in farming practices have created distress for some bird species. Some farmers are now using land once set aside for conservation to plant more corn for use as ethanol. And the disappearance of smaller family farms in favor of larger corporate farms has led to the disappearance of hedgerows -- fences of trees or bushes that reduce erosion and lessen the force of the wind on crops, and at the same time serve as protection and nesting areas for many grassland birds.

Other farms have simply disappeared and been replaced by housing and commercial development, further reducing bird habitat.

The Audubon Society, incorporated in 1905, lists five priorities for Congress to consider to slow the decline of these bird species: reduce global warming, support wetlands, fund ecosystem restoration, ensure biofuels are eco-friendly, and improve conservation programs in the next farm bill.

While federal legislation is an important part of protecting bird habitat, Butcher said individuals can also make a dramatic impact on bird health.

"You don't have to have a lot of land, just a corner of your back yard for native plants," he said.

The berries on native dogwoods, for example, provide a food source for migratory birds. And he suggested that bird lovers not cut down flowers in the fall, instead leaving them up as groundcover for birds in the winter and as a source of seeds. Introducing just a few native plants to perfectly manicured, sterile back yards can make a huge difference, Butcher said.

Other household names on the list of diminishing bird populations are the common tern, ruffed grouse, common grackle and rufous hummingbird.