On a blustery spring day, I crouched behind sagebrush at the edge of the Green River in western Wyoming, waiting for pronghorn to pass by on their northern migration. Occasional snowflakes fluttered into the steel-colored water. I pulled my arms inside my down jacket, zipped to the chin. Hours went by.
Then, across the river, I glimpsed tawny shapes: a dozen pronghorn bunched up on the riverbank, pacing and looking at the current. A doe led the others into the water. At its deepest point, the river carried the swimming pronghorn downstream. The doe scrambled for footing and picked her way through jumbled driftwood at the river’s edge. She leaped up the bank, water streaming from her round belly and thin-boned legs. Once clear of the river, she shook a spray of water from her coat. Then she continued north at a run, the rest of the group trailing after her.
As part of her 120-mile-long migration, this doe had already navigated parts of the Pinedale Anticline gas patch—an intensively drilled piece of public land in western Wyoming—a tricky highway crossing and a couple of subdivisions. Here, along the Green River, she was still 60 miles, a 9,100-foot mountain pass and as much as a month away from her destination, the benchlands along the Snake River in Grand Teton National Park. It’s a challenging journey, and not all of the 300 to 400 pronghorn that attempt it survive.
This pronghorn migration is one of the longest and most dramatic land-animal movements in the United States. That it still happens is remarkable. Around the world, long-distance animal migrations are disappearing as human development blocks and fragments the migration corridors. In the Yellowstone ecosystem of northwest Wyoming and parts of Idaho and Montana, residential and other development has stopped pronghorn from migrating through six of eight historic corridors. Of the two remaining, this route, dubbed the “Path of the Pronghorn” by the biologists who identified it, is the most vulnerable. If it gets cut off, pronghorn will be extirpated in Grand Teton National Park, where snow piles up too deeply for them to survive the winter. In 2008, the U.S. Forest Service granted official protection to the northernmost 45 miles of the corridor, where it crosses the Bridger-Teton National Forest, making it the first federally recognized national migration corridor. But the Green River crossing lies outside that protected stretch.
As it becomes clearer that protecting migration is crucial to maintaining robust wildlife populations, researchers are trying to understand where, how and why wild animals migrate. Advances in tracking technology are providing some answers, but in many ways migration remains a mystery. I thought I might gain some insight if I tried to watch this migration in action—especially in the remote northern part of the corridor, an area only accessible by foot.
Satellite-collar studies have given us a 30,000-foot-high view of the western Wyoming pronghorn migration, and hunters, cowhands and biologists have long wandered stretches of the corridor. But to my knowledge, no one had followed the entire route on the ground. And so, over the course of four seasons, I walked the Path of the Pronghorn. The first autumn, I missed a key drainage and followed the wrong creek to the wrong pass, fighting my way through thick brush and forests without finding so much as a hoofprint. The next spring, I found the right course, but I was a week too late.
The third time, the geography made more sense to me. I remembered how a certain V-shaped valley led to a small notch in a patch of trees at the divide. I could imagine how a fawn that followed its mother along the route a couple of times might memorize the best spots to ford the rivers and scoot through strips of forest, later teaching the course to its own offspring. The animals, however, are elusive, and they travel much faster than a person on foot, especially in the fall. I relied on hoofprints dried into muddy trails, tufts of hair snagged on branches and the occasional distant glimpse of a pronghorn disappearing over a hill. This helped me understand the journey, but I still yearned to see actual animals face their challenges.
The following spring, I drove about 15 miles north of the Green River crossing and parked just inside the forest boundary. The snow was still too deep for ATVs or pickups, and too melted for snowmobilers. The only way in was on foot. This time, however, like the pronghorn, I’d have company.