It's a cold February morning at Elephant Mountain Wildlife Management Area, 25 miles south of Alpine. A 30-person crew stands at attention at headquarters.

From the slopes of the mountain – a retort, like a gunshot. Then, a helicopter appears overhead – and gently places two mule deer does on the ground.

The crew swings into action – giving antibiotic shots, taking blood samples, recording data. Within minutes, the deer – dazed, but placid – are loaded in a Texas Parks and Wildlife trailer. By day's end, they'll have a new home.

Mule deer capture is its own kind of rodeo. Here, it's designed to help struggling deer herds in the Big Bend.

In February 2016, Texas Parks an...