A few decades ago, Bob Spahn of Webster regularly heard the calls of ring-necked pheasants as he did some morning bird watching.

But times have changed. Back then, there were several hundred of the pheasants in the Greater Rochester area. In the last 10 years, however, the bird has all but disappeared from the region. There are generally fewer than five in the yearly bird count, says Spahn, who is the former records chair for the Genesee Ornithological Society. Only one was recorded during the 2006 Christmas Count in the Rochester area.

That's just one of the species that Spahn and other area bird enthusiasts worry about. In fact, declining bird populations are a concern the across the country.

The national Audubon Society recently identified 20 common birds (species with populations over 500,000) whose numbers have declined over the last 40 years. They include grassland birds like the eastern meadowlark, waterfowl such as the greater scaup, and brushland birds like the whippoorwill.

While fewer birds may mean less variety for birdwatchers, there's more at stake. The population of some of the 20 has declined by more than 75 percent, and that doesn't bode well for the environment, says John Waud, an environmental science professor at the Rochester Institute of Technology. Birds are a good indicator of the health of the overall environment, he says, because they use an array of habitats and rely on a variety of food sources.

In addition to the list of 20 birds, the Audubon Society developed state-specific lists of "birds of concern," each having five species. In New York, three of the selected species are not year-round residents: the common tern, grasshopper sparrow, and field sparrow, which are present in the Rochester area only during the summer breeding season. The eastern meadowlark and the ruffed grouse live here year round. On the low end, the ruffed grouse has decreased by 54 percent, according to annual bird counts. At the high end, the eastern meadowlark has lost 72 percent of its population.

The Audubon Society chose the species based on 40 years of data from its Christmas Bird Count and the US Geological Survey's Breeding Birds Survey. Combining the data from the annual counts allowed analysts to compare numbers during two stable periods, winter and summer, when there is no migration.

The decline of the whippoorwill has proved to be a vexing problem for local ornithologists. There have been no breeding birds in the state since the 1980's, says Spahn. The habitat for them exists, he says, particularly in places like the Bergen Swamp, which hasn't changed much over the years.

"People aren't quite sure what's happened," says Spahn.

One theory for the local decline, at least in breeding couples, is a decline in the species of large moths that the whippoorwill feeds its young, says Spahn. The moths are favored because of their high protein content.

The evening grosbeak is another mystery. Before the early 1960's, the birds were scarce.

"It was really exciting to see one," says Spahn. "And it wasn't every year."

Around that time, their numbers erupted. (They were known to crowd out other birds at feeders and eat up entire stockpiles of seed.) But starting in the mid-1980's, they again became scarce. No evening grosbeaks have been recorded in the last decade's worth of local Christmas Bird Counts, and their population nationally has dropped by 78 percent North America, according to the Audubon Society.

In addition to the species identified by Audubon, others have been declining. Nighthawks, for example, have disappeared from Rochester's downtown, says Spahn. That could be tied to changes in city insect populations, he says, or to the phasing out of the gravel roofs the hawks used to nest in.

Crows, blackbirds, and grackles have been hit hard by the West Nile Virus. For 30 years, Spahn has participated in the Breeding Birds Survey, studying the same areas in rural Wayne and Yates Counties each year. He's seen redwing blackbird numbers drop by 25 percent (400 to 300) and grackles by 50 percent. Grackles, which often feed on insects and larvae, are prone to the effects of chemicals such as grub killer, says Spahn.

Grackles made the national Audubon list. Nationwide, their numbers have dropped 61 percent over 40 years (190 million to 73 million).

Although the 20 species have been in decline, some species have been increasing. In the Rochester area, they include bald eagles, robins, and Canada geese. Wild turkey have increased since they first appeared on the Christmas Bird Count in 1993 (there was a single bird that year, but in 2006 there were 83).

While it's important to identify declining species, there's an inherent problem with the Audubon study, says RIT's John Waud. The report shows that bird numbers are going down, but it doesn't comprehensively address the reasons. The organization did stress the impact of habitat destruction and global warming, but Waud says other factors need to be considered as well. Those could include such things as whether there is a declining birth rate or if fewer birds are living longer.

"It's a very important report, but it only has part of the picture," he says. Studies are underway to examine those complex issues, and Waud is involved in one.

Of the birds on the Audubon list, it's the grassland birds that have been hardest hit, and the Rochester region is no exception. Monroe County used to have significant grassland, but much of it was taken for farming or for other uses, such as development. Some grassland has been allowed to grow into forests. And some swamps, another common habitat for birds, have been filled in for development.

"If you look around Monroe County," says Waud, "so many towns are being built out."

Across North America, development and industrialized farming have also had an impact on the environment and bird habitat, says the Audubon Society's analysis.

The report includes nine recommendations to help steady bird populations, and possibly increase them. Among them: preserving farmland, grasslands, forests, and wetlands; patrolling and cleaning up waterfronts; monitoring bird feeders; stopping invasive species, and stopping global warming.

The Audubon's recommendations are "powerful suggestions," says Waud. The dwindling populations appear to be the result of human activity. But like the Audubon Society, Waud says he believes humans also have the ability to maintain populations. And more than likely, if work is done to help preserve one species, it will benefit some of the other species as well, he says.

But there is one other important question to ask, said Waud: Exactly how many of a certain species will be needed to make sure it doesn't become extinct?

"We're never going to get back to that baseline where we're awash in certain species," he says.

The Audubon 20

The Audubon Society report lists these 20 birds, common in North America, whose population decline has been greatest since 1967:

Northern bobwhite: 82 percent

Evening grosbeak: 78 percent

Northern pintail: 77 percent

Greater scaup: 75 percent

Boreal chickadee: 73 percent

Eastern Meadowlark: 72 percent

Common tern: 71 percent

Loggerhead shrike: 71 percent

Field sparrow: 68 percent

Grasshopper sparrow: 65 percent

Snow bunting: 64 percent

Black-throated sparrow: 63 percent

Lark sparrow: 63 percent

Common grackle: 61 percent

American bittern: 59 percent

Rufous hummingbird: 58 percent

Whippoorwill: 57 percent

Horned lark: 56 percent

Little blue heron: 54 percent

Ruffed grouse: 54 percent