For nearly five years and across more than 500 square miles, they counted birds.

Starting in April 2009, scores of Loudoun Wildlife Conservancy volunteers combed suburban neighborhoods, public parks and rural fields in Loudoun County, in search of cardinals, hawks, blackbirds — any winged creature whose presence might reveal something about the changing environment in one of the fastest-growing counties in the nation.

The volunteers finished their tally in December, after more than 64,000 sightings of 259 species of birds, ending the research phase of the Loudoun Wildlife Conservancy’s first bird atlas project, a comprehensive, volunteer-driven effort to create a thorough list of all bird species living in Loudoun.

County wildlife officials hope the research will help create an important guide for residents who are interested in birdwatching and conservation, for scientists who study the changing ecosystem and for officials who consider the environmental effects of development.

“This kind of project involves a lot of work . . . but we decided that because Loudoun was changing so rapidly, and those changes were having a pretty strong impact on the flora and fauna in the county, that we should go ahead and move forward with the atlas,” said Joe Coleman, a Loudoun Wildlife Conservancy founder and president of the Virginia Society of Ornithology.

Most bird atlases are done at the state level about every 20 years and focus on breeding birds. The previous Virginia tally was done in 1985, which meant that the state was due for an update in 2005. But there was no sign then that Virginia was planning to organize another bird atlas imminently, said Spring Ligi, bird atlas coordinator for the conservancy.

“So the Loudoun Wildlife Conservancy decided to step in and do it for themselves,” Ligi said. “We haven’t analyzed our data yet, but we do have some good results in already, and there are clear comparisons to what we saw in 1985.”

One of the most surprising results, Ligi said, is the great diversity of bird species found in Loudoun’s more densely populated eastern suburbs, particularly in Sterling, Ashburn and Brambleton, where much of the county’s development has been concentrated in recent years.

“You generally think of development as being bad for the birds, which is still often true,” Ligi said, “But some of the richest birding areas are actually in these developed areas.”

The abundance of species in those community parks is not necessarily good news, she said, “What we think is that with all this development, the birds are forced into the few areas of habitat, which makes them easier to spot.”

Of the nearly 260 species documented, 103 species were actively breeding in Loudoun, Ligi said. Breeding birds nest in the county and stay during the summer, she said, whereas migrating species, such as yellow-headed blackbirds and long-eared owls, are just passing through.

There are species that come for the winter, Ligi said. This year, birders in Loudoun and throughout the Washington region were thrilled by sightings of snowy owls that appeared as part of a historic mass migration known as an irruption.

“That was so exciting,” Ligi said. But the snowy owl irruption also demonstrates why the bird atlas takes place over five years, she said. “Some species of birds have irruption years, like the snowy owl did, where you won’t see any of them, but then in one year, the population will surge, and you’ll see a ton of them,” she said. “But that would skew the results if the study was just one year, so you have to give it five years to account for those population variations.”

There are success stories. Populations of bald eagles are thriving, Coleman said, and birds that have adapted to suburban and urban settings, including northern cardinals and mockingbirds, are also doing well.

“With some birds, like Carolina wrens, when you have a really rough winter like we just had, they survive in suburban communities where people put out bird feeders,” he said, “and they don’t survive in rural areas where they survive by foraging for their own food.”

Some species that depend on a rural environment have not fared as well. The bird atlas shows dwindling numbers of the northern bobwhite and the cerulean warbler, which has been considered for listing as a threatened or endangered species, he said.

Warblers depend on healthy riparian buffers — thick areas of vegetation bordering streams — to survive, Coleman said. Those buffers are often compromised by development, a fact he hopes the bird atlas might change.

“What we’re hoping is that the information we’re pulling together will help convince people that some of these protective steps are worth taking,” he said.

In the coming months, the data collected by volunteers will be analyzed by the Breeding Bird Atlas Explorer, an online system hosted by the Patuxent Wildlife Research Center. A report, including distribution maps of local bird populations, will be published next year, along with updated pamphlets and bird species lists for visitors to the county’s nature centers, Ligi said.

“We want to identify the critical areas that we should focus on protecting,” Ligi said, “And we want to create something that the general birdwatcher would be able to use, too.”

The Loudoun Wildlife Conservancy will hold a series of birding walks at local parks from May 3 to 11, in recognition of International Migratory Bird Day, which is May 10. All walks begin at 8 a.m. and require registration. Information is at