BOISE, Idaho - Environmentalists urged a federal judge Monday to reverse a 2005 U.S. Fish and Wildlife Service ruling that kept the sage grouse off the endangered species list, arguing the agency's decision ignored science and was tainted by political meddling.

For decades, populations of the chicken-sized bird that once thrived across more than 150 million acres of Western sagebrush in 13 states and Canada have been in decline. The case could have broad implications for all kinds of activities on public lands across the West.

Conservationists and biologists say threats to the bird's habitat from urban growth, oil and gas drilling, ecosystem degradation caused by wildfire, road building and West Nile virus have reduced its present population to a fraction of its historic numbers, estimated by some at more than 1 million in the early 1800s.

Despite those declines, Fish and Wildlife ruled against granting the highly prized game bird endangered species status, a decision that pleased energy companies and ranchers who feared federal protections would crimp oil and gas projects in states including Wyoming and Montana and restrict grazing from Washington to Colorado.

Last summer, the Western Watersheds Project, an Idaho-based conservation group, challenged the agency's decision in a lawsuit filed in U.S. District Court in Boise.

The group's lawyer, Laird Lucas, claimed agency decision makers disregarded research compiled by Western state biologists showing significant population declines in the last 40 years, continued threats to the sagebrush ecosystem and an uncertain future for the grouse.

In Monday's hearing, Lucas also complained that political pressure from top-level Bush administration appointees trumped science and reason. Lucas singled out Julie MacDonald, a former Interior Department deputy assistant for fish, wildlife and parks who resigned in May amid complaints she bullied federal scientists and shaped policy favorable to administration priorities.

Lucas provided e-mails and memos from MacDonald, underscoring her role in downplaying scientific conclusions on the bird's populations and habitat.

"The record is filled with political interference and overriding science," Lucas argued. "They ought to have let the experts do what they were supposed to do."

U.S. District Judge B. Lynn Winmill gave no indication when he would rule.

Because the sage grouse is skittish and sensitive, oil and gas companies that have been enjoying a boom in recent years in Wyoming, Utah and Montana already tailor their production and exploration schedules around the mating season.

Industry executives acknowledge that future drilling in places such as Colorado's Roan Plateau and the Green River Basin in Utah and Wyoming hinges on future federal protections as well as their own efforts to lessen their footprint in sensitive sage grouse habitat.

Michael Wigmore, an attorney for Anadarko Petroleum Corp., one of several parties helping defend the federal government, said it's easy to find data that contradict federal decisions involving species protection.

Wigmore, along with lawyers representing the states of Wyoming and Idaho, claimed there is no evidence to support listing and that extensive state conservation programs have helped either stabilize or increase sage grouse populations in the last 20 years. The most recent population figures range from 150,000 to as many as 500,000 grouse across an 11-state area.

"The risks have been increasing, but so have the bird populations," said Robert Nicholas, a deputy Wyoming attorney general.

Other parties intervening in the lawsuit on behalf of the Fish and Wildlife Service include the Petroleum Association of Wyoming, the Idaho Farm Bureau, the Oregon Livestock Producers Association and Nevada Farm Bureau.