The two-way radio crackled to life, “We’re coming in with three ewes and a young ram.”

Two minutes later, a helicopter appeared on the horizon, four large nylon bags slung below its belly. It touched down in a rocky basin, gently so as not to jostle its delicate cargo. A dozen workers and volunteers rushed forward to release the bags, each one holding a blindfolded desert bighorn sheep.

These wild sheep, captured not far from Las Vegas by the Nevada Department of Wildlife, were destined for release in the Grand Staircase-Escalante National Monument in southern Utah. Relocations like this one are part of a far-reaching conservation plan to help re-establish bighorn sheep in their historic range across the West.

Bighorns are descended from wild Siberian sheep that crossed the Bering land bridge to North America about 100,000 years ago. These herds spread southward, diversifying and adapting to local habitats.

Bighorn sheep – named for their immense, curling horns, which can weigh up to 30 pounds – inhabit steep, barren terrain that few other species can tolerate.

Thanks to their hardiness, bighorn sheep have long been a symbolic species. Early Native Americans carved their likenesses into rocks, and the first settlers embraced them as symbols of the rugged wilderness. At their peak, more than 2 million bighorns roamed the West, gracefully cavorting on rocky hillsides from California to Nebraska.

But by the late 19th century, bighorn sheep were in trouble. The domestic sheep industry had taken hold in the West, and wild sheep had no immunity against diseases introduced by European livestock. As millions of domestic sheep inundated the landscape, deadly pathogens such as scabies and pneumonia decimated the bighorn population. Unregulated hunting took a toll on the few wild herds that remained.

By 1940, the bighorn population had plummeted to fewer than 20,000, isolated in tiny enclaves scattered across the Western states.

In recent decades, state wildlife management agencies have undertaken extensive conservation work to help bring bighorn sheep back from the brink. Much of the work focuses on capturing bighorns from successful herds and relocating these sheep to other areas where the species once thrived.

To find those areas, state agencies looked to an array of sources, from archaeological studies of Native American hunting grounds to the expedition notes of Lewis and Clark.

Biologists then carried out surveys to assess whether the areas could still support wild sheep. Highways, housing developments and solar fields now dominate much of the once rugged Western landscape, leaving large areas uninhabitable for bighorns. Elsewhere, mining has polluted water sources needed to sustain big game.

Once wildlife managers pinpointed habitats where bighorns could still thrive, “it was just a matter of organizing our ability to figure out how to successfully and humanely capture those animals,” said Mike Cox, a staff biologist with the Nevada Department of Wildlife. But success has brought risks as well. Booming bighorn populations have increased the possibility of contact with livestock, putting the wild sheep again at risk for disease.

So the Nevada wildlife agency is careful to leave large buffer zones – ideally at least 20 miles – between the transplanted bighorns and the 70,000 domestic sheep scattered across the state.

But the movements of wild and domestic sheep are not always predictable, and the spread of disease can be difficult to stop.

“Without a doubt,” said Kevin Hurley, the conservation director of the Wild Sheep Foundation, a nonprofit group, “one of the biggest challenges we have is this whole contact issue between domestic sheep and goats and wild sheep.” Surprisingly, many of the nonprofit groups that champion the conservation of bighorn sheep, including the Wild Sheep Foundation and the Fraternity of the Desert Bighorn, are made up almost entirely of hunters. Hunting bighorn sheep and other big game is legal in Nevada and other Western states, but hunting opportunities are meted out carefully by state agencies based on annual population estimates.

“It truly is hunters and fishermen that pay the cost of wildlife management,” Hurley said. “It’s a way that they can put something back into the resource that maybe they’ve hunted, maybe they haven’t, maybe they’d like to.”

Although partnerships with hunters have made possible the conservation of bighorn sheep and other big-game species, some experts worry that such heavy reliance on hunters is unsustainable over the long term. If interest in hunting dwindles, these experts warn, conservation projects may need a broader base of financial and logistical support.

“What we need to do is get the public committed,” said Karen Layne, a member of the Nevada Board of Wildlife Commissioners. “It’s the general population’s wildlife, and that general population – the nonhunters – needs to be involved in this process as well.”

But conservation advocates say there is “a lot of reason to be optimistic and hopeful,” as Hurley put it.