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CORVALLIS, Ore. -- Roads and petroleum wells in Wyoming's oil and gas country don't seem to interfere with the nesting of ferruginous hawks, according to recent findings by Oregon State University wildlife researchers.

In their three-year study, published in the journal PLOS ONE, wildlife biologists Zach Wallace and Patricia Kennedy found that the birds were equally likely to return to nests near energy infrastructure, such as roads and well pads, as to those farther away.

The birds' nesting choices proved to be influenced more by abundance of prey animals such as ground squirrels, and by relatively sparse sagebrush cover, than by structures associated with oil and gas fields, the researchers concluded.

The study, conducted in collaboration with the U.S. Forest Service and Wyoming Department of Game and Fish, is the largest in the U.S. so far on the impacts of oil and gas development on the federally protected hawks, which are regarded as a "species of conservation concern" by some federal and state agencies.

But it's too early, Wallace cautioned, to assume that oil and gas activities are benign.

"We don't have pre-construction data," he said, "so we were studying birds that had continued to nest after energy exploration began. It is possible that some hawks may already have abandoned the areas of densest development prior to our study."

Kennedy said the long-term effect of energy development on abundance of prey is unknown.

"We know from the literature that ferruginous hawks can nest in working landscapes," she said. "But we present our findings with some caution, because we don't know what the thresholds are," for habitat changes that will harm the birds' reproductive success.

"Some prey species seem to thrive under disturbances from oil and gas development; others may not."

Kennedy is a professor in OSU's College of Agricultural Sciences stationed at the Eastern Oregon Agricultural Research Center in Union, Ore. Wallace led the study as Kennedy's master's student and now works for Eagle Environmental, a conservation consulting firm in New Mexico.

The ferruginous hawk (Buteo regalis) is the largest hawk species in North America. The birds are partially migratory, wintering as far south as central Mexico and returning north in the spring to breeding territories in the arid shrub- and grasslands of the western U.S.

The hawks nest in trees and rocky outcrops, returning to prior years' nests if these are available. They also nest readily on human-made structures such as artificial nesting platforms, power poles, abandoned windmills, even gas condensation tanks. They will nest on the ground if elevated structures are not present, Kennedy said.

Birds that inhabit grasslands and shrublands are declining around the world primarily because of human-caused disturbances, Wallace said. He and Kennedy undertook the study to determine which of several key influences were most important in the hawks' reuse of breeding territories and nesting success: abundance of prey, shrub cover, weather, type of nest substrate, and density of human structures such as roads and well pads.

The researchers counted hawk nests from a small airplane over three seasons, and they sampled prey species on the ground. Their study area covered nearly half the state of Wyoming and included both public and private land.

They divided the sampling territory into areas with low, medium and high density of oil and gas infrastructure. After the initial nest count, they monitored the nests during spring breeding season over the next two years to see whether the birds returned to prior years' nests and how many young they produced.

Based on earlier research, they expected that returning birds would avoid nests within 1.5 kilometers of roads and well pads. Instead, they found that the birds were equally likely to come back to these nests as to the ones farther away.

The findings could affect the mitigation measures required of energy companies to protect wildlife habitat, said Wallace, which are now negotiated with land management authorities on a project-by-project basis.

"One of the strengths of our study is its broad spatial scale, which makes it more relevant to management decisions than the smaller-scale studies that have been done in the past," he said. "We were able to study these hawks at the scale of their ecology, and also at the scale of oil and gas development."

Wyoming's oil and gas industry has grown rapidly since the late 20th century, although growth has slowed lately as prices for fossil fuels have declined.

"We collected an excellent, large dataset on the hawks' nesting behavior in both disturbed and undisturbed areas," Wallace said. "This study lays the groundwork for rigorous before-and-after studies if and when oil and gas drilling spreads into now-undeveloped areas."