The route there is about a quarter-mile wide, but sometimes narrows to a single-file path. And the land was for sale. If bought by developers, the migration route would most likely be blocked.

Buying it made sense to the coalition. “This is one of the last, best migrations,” said Luke Lynch, state director of the Conservation Fund, which is in the process of buying the 364 acres for about $2 million. The Knobloch Foundation is providing half of that; the rest will have to be raised.

A longer-term goal of the coalition is to change public-land management to place greater value on migration routes. The Bureau of Land Management is revising plans for land use in an area that includes the Red Desert. If the bureau designates the migration route for protection, that could serve as a precedent for other decisions by the bureau and other agencies.

That, Mr. Sharkey says, is “the big enchilada.”

Meetings with the bureau are continuing. The conservationists have the support of the Wyoming Game and Fish Department, which has jurisdiction over the animals, but not the land. A draft report is due later this year.

The conservationists and scientists will face other challenges in trying to protect the migration route. But more accurate data provided by new technology and the deer themselves — even if they are reluctant partners in the helicopter lifts, teeth-pulling and other tests — are making political and regulatory action easier, mostly by pinpointing the small targets of land that are important for protecting the route.

“It’s not about huge vast landscapes,” said Peter Aengst, senior director of the Northern Rockies region for the Wilderness Society, which is part of the mule-deer conservation effort. “It’s about very narrow but very critical acres.”