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BILLINGS, Mont. (AP) — Oil and gas development in the Western U.S. could continue to cause sage grouse numbers to decline despite limits on drilling meant to protect the struggling bird species, according to scientists.

Researchers from the U.S. Geological Survey and Colorado State University reached the conclusion after examining the effects of drilling on greater sage grouse over a 25-year period ending in 2008.

They found that populations of the chicken-sized bird dropped 14 percent annually in areas with at least 10 oil or gas wells per square mile.

Federal land management rules recently crafted to protect grouse across their 11-state range would allow that many wells or more in areas crucial to the birds’ long-term survival.

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Populations were stable when no wells were present, the researchers concluded in their findings published in The Journal of Wildlife Management.

Kathleen Sgamma with the Western Energy Alliance industry group said the researchers ignored changes to federal and state land use policies meant to concentrate drilling in some areas and avoid impacting the most sensitive sage grouse habitat.

Study co-author Cameron Aldridge said more concentrated drilling may reduce impacts. But that doesn’t mean it won’t affect grouse. He said more power lines, pipelines, vehicles, noise and other human activity are associated with multiple oil or gas wells than there would be with a single well.

“It’s not a well or well pad that causes a decline. It’s all of the associated activity that contributes collectively,” said Aldridge, an associate professor at Colorado State University’s Department of Ecosystem Science and Sustainability.

Wyoming Game and Fish Department sage grouse coordinator Tom Christiansen said limited energy development is expected in the grouse’s core habitat for the foreseeable future.

The U.S. Interior Department last year declined to put sage grouse on the list of endangered and threatened species despite a long-term decline blamed on oil and gas development, grazing, wildfires, residential development and disease.

The agency cited in part the limits on drilling targeted at preserving the bird’s breeding grounds in areas of Wyoming, Montana, Colorado and other states. Under those rules, companies can construct one well pad per square mile, with each pad containing multiple wells.

Interior spokeswoman Jessica Kershaw said in an emailed statement that the agency was reviewing the new study.

Land management plans adopted by the agency last year include population monitoring efforts that would trigger additional conservation measures should grouse numbers or habitat quality decline, she said.

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Associated Press writer Mead Gruver contributed to this story.