High on the Mexican Plateau on a stormy March day, fierce winds blow across the vast, flat Chihuahuan Desert grasslands and the distant peaks of the Sierra Madre. A group of scientists from U.S. and Mexican NGOs and universities unfurls a long net for catching birds and stakes it into the ground. A grasshopper sparrow tagged with a transmitter is around here somewhere, and we aim to catch it.

The bird, nicknamed Frequency 227, blends into the dust-brown backdrop. It and the other 13 Baird's and grasshopper sparrows on our list are the color of mud and sand, flecked with white, orange and yellow. They evolved with this vast grassland -- which once stretched from the northern and central states of Mexico into southern New Mexico, Texas and Arizona -- foraging each winter in grass grazed by bison, and hiding from predators in taller grass. Come spring, they flew to Montana, Wyoming, the Dakotas and Canada to breed. Ninety percent of the approximately 29 grassland bird species breeding in the Western Great Plains -- including these sparrows -- winter in the Chihuahuan Desert.

Since the 1960s, however, grassland bird populations have plummeted, some species by as much as 85 percent -- the sharpest decline of any group of North American birds. Researchers have long assumed they were most vulnerable while reproducing, so they've focused on the expansion of agriculture, urbanization, energy development and other factors destroying habitat in the birds' northerly haunts. But Arvind Panjabi, my guide and the international program director at the Rocky Mountain Bird Observatory in Fort Collins, Colo., wants to know: What if the Chihuahuan Desert grasslands are equally critical?

In an earlier study of vesper sparrows here, near the northern Chihuahua town of Janos, Panjabi found that only a quarter to a half survived the winter. Those that lived spent more time around tall grass, which is increasingly rare due to over-grazing and drought. At the lower end of that mortality range, sustaining the population would require each vesper pair to raise eight chicks per year, rather than their typical three to four, Panjabi says. And that's excluding predation and die-offs in other seasons.

So besides tracking the survival rates of the sparrows we're after today, Panjabi is trying to identify the features -- tall grass, hills -- most important to their survival, to make conservation efforts more effective.

Work like Panjabi's is increasingly urgent. Pronghorn, kit fox, badger, mule deer, prairie dogs, Aplomado falcons and golden eagles also rely on these arid grasslands, which have been decimated over the last two decades by drought, overgrazing and sodbusting. When I visit, the roadsides are lined with the bones of starved cows.

Many struggling ranchers have been selling their land to farmers. In central and northern Chihuahua, those are mostly Mexican Mennonites, themselves so pinched by drought and desperate for new land as their wells dry up and their population swells that they have been known to buy ranches for two to three times their value. Once plowed, however, the habitat is nearly useless to wildlife, the grasses' deep roots gone and crucial plant biodiversity permanently lost.

Worse, grassland conversion appears to be escalating in the flat expanses where most birds live. A survey of satellite images of Valles Centrales, a Massachusetts-sized area southeast of Janos, showed that farmers converted over 170,000 acres -- more than 60 percent -- of its flat grasslands between 2006 and 2011, and almost 35 percent of all its grasslands. At current rates, flat grasslands will disappear from Valles Centrales by 2025.

In hopes of saving some of what's left, U.S. and Mexican conservationists are trying to help ranchers stay on their land and steward it better through more sustainable grazing practices. But the biggest challenge is reaching the more insular, and influential, Mennonite community. Still, when I talk to Panjabi at El Uno, a 46,000-acre Nature Conservancy-owned ranch near Janos, he's optimistic. "The story is not over," says the bearded 42-year-old mandolin player one night over a dinner of refried beans and menudo, a soup made with beef stomach. "There are still a lot of grasslands."