Editor's note: This imperiled pronghorn migration is historic and the longest overland mammal migration in the Lower 48 (second only to caribou in North America). Energy development, human encroachment, highway mortality and other factors have led to a significant decline in the pronghorn's population.

In fall 2011, Jeff Burrell was sitting with his spotting scope at Trappers Point, a 6,000-year-old pronghorn migration corridor in Sublette County, watching the fleet-footed ungulates nervously approaching busy Highway 191.

The pronghorn clearly looked flummoxed and scared when faced with the obstacle on their path from Grand Teton National Park to winter range in south central Wyoming.

“They’d be about a mile away and you could see that they were on high alert,” Burrell said, standing near the same vantage point this October.

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