BILLINGS, Mont. (AP) — Scientists say 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.

Researchers from the U.S. Geological Survey and Colorado State University examined the effects of drilling on greater sage grouse over a 25-year period. They found that populations of the chicken-sized bird dropped 14 percent annually in areas of Wyoming with at least 10 oil or gas wells per square mile.

Federal 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. Wyoming sage grouse coordinator Tom Christiansen says limited energy development is expected in the grouse's core habitat for the foreseeable future.