A new study has documented what many birdwatchers, sportsmen and those who listen to birdsong outside their windows have noticed for years: many of America's once-common birds are becoming uncommon.

 They are not in danger of dying out, but some of our most familiar and loved birds are sharply declining in numbers and are in trouble, according to a study by the National Audubon Society.

The list includes birds that frequent backyard feeders, birds from the wild north country and birds popular with hunters.

Among the birds in trouble: the ruffed grouse, Pennsylvania's state bird whose numbers have dropped by 54 percent across northern states since 1967.

The study looked at 40 years worth of bird counts done at various times of the year annually by millions of volunteer birdwatchers around North America.

But the slippage has been so gradual for so long that it's been easy to overlook.

The threats, according to NAS: sprawl, global warming, pressure to plant land set aside for conservation into corn for ethanol, and the spread of industrialized intensive agriculture that results in plains of bare, insectless fields.

The good news: "We have the time and the tools to turn things around," Scott Weidensaul, a well-known ornithology author and Schuylkill County resident told reporters during a teleconference last week.

Weidensaul, who grew up in the coal country of eastern Pennsylvania, rhapsodized about awakening on mornings to the whistling of bobwhite quail and the roller coaster calls of whip-poor-wills.

"The song of the eastern meadowlark used to be a soundtrack of summer," he said. "Today, you can't find a bobwhite in Pennsylvania and it's a red-letter day for a birder to find a whip-poor-will."

Pressures on agriculture and development seem to have hurt grassland birds the most, closely followed by shrub, wetland and forest-dependent species, said Greg Butcher, bird conservation director for Audubon and chief author of the study.

Climate change from global warming seems already to be affecting some bird species when they try toshift their range north because of warmer temperatures but are trapped by the lack of "bridge" habitat, NAS officials said.

In Pennsylvania, you can add too many deer to the causes of declines in birds that depend on forest habitat, Weidensaul said.

"We have created a perfect world for whitetail deer and they're just munching their way through it."

One reason grouse are becoming more scarce is that the understory growth they depend on to hide and for insects to eat just isn't there because of deer browsing, he said.

Weidensaul echoed my thoughts when he noted that as a hunter he had long assumed game species were safe from such population crashes because of all the conservation efforts and money poured into wildlife management.

Yet, here we have grouse, quail, northern pintail and greater scaup ducks on Audubon's Top 20 list of common birds that have lost at least half their population in the last four decades.

"No species is safe from the sweeping landscaping changes we're facing," Weidensaul concluded.

Then what can we do about it?

Plenty, say the experts.

Don't overestimate the good that can come from doctoring even a small corner of your yard to aid birds. Put in native plants that bear berries and seeds, advises Weidensaul.

The Pennsylvania chapter of NAS recently added a coordinator for its Important Bird Areas in southeastern Pennsylvania to work with landowners willing to manage their properties to aid birds.

Things like farmers delaying cutting of hay until mid-July would enable grass-nesting species to fledge young ones from nests, notes Tim Schaeffer, the state chapter's executive director.

"We're looking to build relationships here in Pennsylvania," Schaeffer said.

To contact Kim Van Fleet, IBA coordinator for this part of the state, call her at 213-6680, extension 11.

Residents also were urged to push their legislators to retain land-conservation programs under threat in the Farm Bill currently being debated in Congress.

And residents were advised to reduce the global-warming threat by practicing conservation at home and by urging local, state and federal legislators to adopt policies to cap greenhouse emissions.

For more on the study and how to help the birds, go to the NAS Web site, www.audubon.org.