CHEYENNE, Wyo. >> A judge has halted plans for oil and gas drilling on vast areas of Wyoming and Montana, citing concerns about a sagebrush-dwelling bird.

The U.S. Bureau of Land Management didn’t adequately consider how the drilling would affect the greater sage grouse, nor an option to defer drilling in the bird’s prime habitat, Idaho U.S. District Judge Ronald E. Bush ruled Wednesday.

Bush ordered more study of potential effects on the bird before drilling may proceed.

The drilling would occur on over 600 square miles of federal land scattered across the energy-rich states. The Bureau of Land Management auctioned off hundreds of leases in sage grouse habitat in four sales in 2017.

Sage grouse are a chicken-sized, primarily ground-dwelling bird whose numbers have fallen significantly from the millions that inhabited the U.S. West in frontier times. The U.S. Fish and Wildlife Service determined in 2010 that the bird deserved special protection but said in 2015 that conservation efforts led by Wyoming made that unnecessary.

The environmental group that sued over the leases praised Bush’s ruling.

“This ruling sends a very strong message that the BLM can no longer lease public lands for fossil fuel development without weighing the outcomes for sensitive lands and wildlife,” Erik Molvar, executive director of Western Watersheds Project, said in a statement today.

BLM spokesman Brad Purdy declined to comment, citing agency policy not to discuss ongoing litigation. The agency’s allies in the case included the Western Energy Alliance industry group and the state of Wyoming, where Republican Gov. Mark Gordon was weighing whether to appeal.

“The governor is dismayed by Judge Bush’s ruling but is pleased that the leases have not been vacated,” Gordon spokesman Michael Pearlman said by email.

The ruling comes amid a federal oil and gas leasing moratorium imposed by President Joe Biden’s administration while it studies the effects on climate change.